Faster string serialization by optimising packInt

[batterseapower]

We can avoid the loop and calling Integer.numberOfLeadingZeros very easily when writing small integers. This helps serialization of ASCII strings quite a bit.

[batterseapower]

Travis build seems to be failing due to an OOM rather than an actual problem with this change?

[jankotek]

Travis builds are bit unstable, dont think this change is causing build error.

I am going to use this code, but STRING serializer will gets its long-pack method with this improvement.

[jankotek]

Nice, microbenchmarks shows it is almost 3x faster:

Benchmark                         Mode  Cnt         Score        Error  Units
StringReadWrite.writeUTF         thrpt  200   3862070.314 ± 114229.614  ops/s
StringReadWrite.writeUTFShorter  thrpt  200  11174539.297 ± 427621.738  ops/s

https://github.com/jankotek/mapdb-microbenchmarks/blob/master/src/main/java/org/mapdb/jmh/StringReadWrite.java

I merged bit modified version, but its now on by default.