Reduce memory footprint by reusing buffers

[cezarsa]

This PR adds some benchmarks for Collection.Insert() and changes mongoSocket.Query() and addBSON() to reuse allocated []byte buffers by getting and putting then into a sync.Pool.

Also the function MarshalBuffer was added to the bson package. It behaves in the exact same way as Marshal but instead of allocating a []byte buffer it receives an already existing buffer.

I came upon these changes after profiling a real world application that inserts a lot of documents simultaneously.

The added benchmarks show a really nice decrease in memory footprint specially when large messages are being sent to MongoDB, as is the case of inserting multiple documents at once:

$ benchcmp before.txt after.txt
benchmark                     old ns/op     new ns/op     delta
BenchmarkInsertSingle-4       94191         90779         -3.62%
BenchmarkInsertMultiple-4     1076423       937569        -12.90%

benchmark                     old allocs     new allocs     delta
BenchmarkInsertSingle-4       50             46             -8.00%
BenchmarkInsertMultiple-4     456            242            -46.93%

benchmark                     old bytes     new bytes     delta
BenchmarkInsertSingle-4       2472          1320          -46.60%
BenchmarkInsertMultiple-4     191180        29228         -84.71%

I did't use gocheck for the benchmarks because it was returning some weird numbers. I'm not sure whether I did something wrong but these are the numbers for the exact same code only adapting it to use gocheck:

$ go test -check.f BenchmarkInsert -check.b -check.bmem
PASS: session_test.go:4191: S.BenchmarkInsertMultiple        500       2627615 ns/op      813412 B/op       2612 allocs/op
PASS: session_test.go:4175: S.BenchmarkInsertSingle     5000        265921 ns/op       16268 B/op        214 allocs/op
OK: 2 passed
PASS
ok      gopkg.in/mgo.v2 3.462s

Compared to the use of native benchmarks:

$ go test -check.f xxx -test.bench BenchmarkInsert -test.benchmem
OK: 0 passed
PASS
BenchmarkInsertSingle-4        20000         99943 ns/op        1384 B/op         48 allocs/op
BenchmarkInsertMultiple-4       2000       1101579 ns/op       32590 B/op        343 allocs/op
ok      gopkg.in/mgo.v2 5.449s

Maybe I did something wrong and I'm open to work on it further if you have any tips.

[cezarsa]

Today I figured a way to simplify the code and improve even more the benchmarks. I've already updated the commits and the benchmark result comparison on the original comment.

[niemeyer]

That's nice. I've been meaning to look into buffer reuse, but always thought about the harder case which is unmarshaling. This is a much easier win.

Will need a few tweaks since we should expose that via an interface such as bson.NewEncoder instead of MarshalBuffer, but I can tweak that on my end.