Use new context-based API for blosc benchmark to fix hanging

[jack-pappas]

I compiled the blosc benchmark program on multiple systems with varying hardware configurations and under both Windows and Linux; then I ran the suite benchmark, specifying various numbers for the thread count (based on the number of cores in a particular system). Each time, the program ran fine for 1 and 2 threads, then hung once it reached the Compression Level 0 step for 3 threads. Strangely, I was able to run the single benchmark just fine for larger numbers of threads (e.g., 4).

Out of curiosity, I modified the bench program so it used the new context-based APIs instead of the older "global" functions and the problem disappeared -- I'm able to run the suite benchmark without issue now. This PR includes the changes I made to the benchmark program to use the new context-based APIs.

[FrancescAlted]

Yes, I can reproduce that in my Linux box. But as I said before, the '_ctx()' is not the 'new' API, and besides, your PR is breaking the test suite in TravisCI. This should be some artifact introduced after 1.5.0. @crspeller do you have some suggestion on this one?

[FrancescAlted]

Tested with clang (3.3 and 3.5) and GCC (4.9.1) on Ubuntu 14.10 and all exposes the problem.

[jack-pappas]

I spoke too soon, perhaps, when I said these changes fixed the threading problem. After I submitted this PR, I realized I had only tried running the bench program using blosclz. I tried running it with zlib instead and ran into threading issues again. This time, instead of hanging, the benchmark ran into some problem and crashed:

Blosc version: 1.5.3.dev ($Date:: 2014-12-30 #$)
List of supported compressors in this build: blosclz,lz4,lz4hc,snappy,zlib
Supported compression libraries:
  BloscLZ: 1.0.3
  LZ4: 1.5.0
  Snappy: 1.1.1
  Zlib: 1.2.8
Using compressor: zlib
Running suite: suite
--> 1, 2097152, 8, 19, zlib
********************** Run info ******************************
Blosc version: 1.5.3.dev ($Date:: 2014-12-30 #$)
Using synthetic data with 19 significant bits (out of 32)
Dataset size: 2097152 bytes Type size: 8 bytes
Working set: 256.0 MB       Number of threads: 1
********************** Running benchmarks *********************
memcpy(write):        121.9 us, 16410.3 MB/s
memcpy(read):         162.5 us, 12307.7 MB/s
Compression level: 0
comp(write):      162.5 us, 12307.7 MB/s      Final bytes: 2097168  Ratio: 1.00
decomp(read):     121.9 us, 16410.3 MB/s      OK
Compression level: 1
comp(write):     9140.6 us, 218.8 MB/s    Final bytes: 171752  Ratio: 12.21
decomp(read):    2478.1 us, 807.1 MB/s    OK
Compression level: 2
comp(write):     8815.6 us, 226.9 MB/s    Final bytes: 121560  Ratio: 17.25
decomp(read):    2112.5 us, 946.7 MB/s    OK
Compression level: 3
comp(write):     8896.9 us, 224.8 MB/s    Final bytes: 121528  Ratio: 17.26
decomp(read):    2112.5 us, 946.7 MB/s    OK
Compression level: 4
comp(write):     11456.3 us, 174.6 MB/s   Final bytes: 80000  Ratio: 26.21
decomp(read):    2356.3 us, 848.8 MB/s    OK
Compression level: 5
comp(write):     11700.0 us, 170.9 MB/s   Final bytes: 74704  Ratio: 28.07
decomp(read):    2356.3 us, 848.8 MB/s    OK
Compression level: 6
comp(write):     13000.0 us, 153.8 MB/s   Final bytes: 50636  Ratio: 41.42
decomp(read):    2193.8 us, 911.7 MB/s    OK
Compression level: 7
comp(write):     13040.6 us, 153.4 MB/s   Final bytes: 37396  Ratio: 56.08
decomp(read):    2356.3 us, 848.8 MB/s    OK
Compression level: 8
comp(write):     15478.2 us, 129.2 MB/s   Final bytes: 37094  Ratio: 56.54
decomp(read):    2315.6 us, 863.7 MB/s    OK
Compression level: 9
comp(write):     14708.9 us, 136.0 MB/s   Final bytes: 29485  Ratio: 71.13
decomp(read):    2437.5 us, 820.5 MB/s    OK
--> 2, 2097152, 8, 19, zlib
********************** Run info ******************************
Blosc version: 1.5.3.dev ($Date:: 2014-12-30 #$)
Using synthetic data with 19 significant bits (out of 32)
Dataset size: 2097152 bytes Type size: 8 bytes
Working set: 256.0 MB       Number of threads: 2
********************** Running benchmarks *********************
memcpy(write):        121.9 us, 16410.3 MB/s
memcpy(read):         162.5 us, 12307.7 MB/s
Compression level: 0
comp(write):      162.5 us, 12307.5 MB/s      Final bytes: 2097168  Ratio: 1.00
decomp(read):     203.1 us, 9846.2 MB/s   OK
Compression level: 1
comp(write):     3575.0 us, 559.4 MB/s    Final bytes: 171752  Ratio: 12.21
decomp(read):    1462.5 us, 1367.5 MB/s   OK
Compression level: 2
comp(write):     3006.3 us, 665.3 MB/s    Final bytes: 121560  Ratio: 17.25
decomp(read):    1259.4 us, 1588.1 MB/s   OK
Compression level: 3
comp(write):     3168.8 us, 631.2 MB/s    Final bytes: 121528  Ratio: 17.26
decomp(read):    1300.0 us, 1538.5 MB/s   OK
Compression level: 4
comp(write):     5606.3 us, 356.7 MB/s    Final bytes: 80000  Ratio: 26.21
decomp(read):    1381.3 us, 1448.0 MB/s   OK
Compression level: 5
comp(write):     5403.1 us, 370.2 MB/s    Final bytes: 74704  Ratio: 28.07
decomp(read):    1421.9 us, 1406.6 MB/s   OK
Compression level: 6
comp(write):     6256.3 us, 319.7 MB/s    Final bytes: 50636  Ratio: 41.42
decomp(read):    1381.3 us, 1448.0 MB/s   OK
Compression level: 7
comp(write):     6784.4 us, 294.8 MB/s    Final bytes: 37396  Ratio: 56.08
decomp(read):    1543.8 us, 1295.5 MB/s   OK
Compression level: 8
comp(write):     8043.8 us, 248.6 MB/s    Final bytes: 37094  Ratio: 56.54
decomp(read):    1462.5 us, 1367.5 MB/s   OK
Compression level: 9
comp(write):     14665.7 us, 136.4 MB/s   Final bytes: 29485  Ratio: 71.13
decomp(read):    2478.1 us, 807.1 MB/s    OK
--> 3, 2097152, 8, 19, zlib
********************** Run info ******************************
Blosc version: 1.5.3.dev ($Date:: 2014-12-30 #$)
Using synthetic data with 19 significant bits (out of 32)
Dataset size: 2097152 bytes Type size: 8 bytes
Working set: 256.0 MB       Number of threads: 3
********************** Running benchmarks *********************
memcpy(write):        121.9 us, 16410.3 MB/s
memcpy(read):         162.5 us, 12307.7 MB/s
Compression level: 0
comp(write):      203.1 us, 9846.0 MB/s   Final bytes: 2097168  Ratio: 1.00
decomp(read):     203.1 us, 9846.2 MB/s   OK
Compression level: 1
comp(write):     4590.6 us, 435.7 MB/s    Final bytes: 171752  Ratio: 12.21
decomp(read):    1259.4 us, 1588.1 MB/s   OK
Compression level: 2
comp(write):     5565.6 us, 359.3 MB/s    Final bytes: 121560  Ratio: 17.25
decomp(read):    1137.5 us, 1758.2 MB/s   OK
Compression level: 3
comp(write):     5525.0 us, 362.0 MB/s    Final bytes: 121528  Ratio: 17.26
decomp(read):    1096.9 us, 1823.4 MB/s   OK
Compression level: 4
comp(write):     5321.9 us, 375.8 MB/s    Final bytes: 80000  Ratio: 26.21
decomp(read):    1340.6 us, 1491.8 MB/s   OK
Compression level: 5
comp(write):     5484.4 us, 364.7 MB/s    Final bytes: 74704  Ratio: 28.07
decomp(read):    1340.6 us, 1491.8 MB/s   OK
Compression level: 6
comp(write):     7596.9 us, 263.3 MB/s    Final bytes: 50636  Ratio: 41.42
decomp(read):    1665.6 us, 1200.7 MB/s   OK
Compression level: 7
comp(write):     7596.9 us, 263.3 MB/s    Final bytes: 37396  Ratio: 56.08
decomp(read):    1665.6 us, 1200.7 MB/s   OK
Compression level: 8
comp(write):     8287.5 us, 241.3 MB/s    Final bytes: 37094  Ratio: 56.54
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