Increase bson.NewObjectId() performance by caching the process id

[256dpi]

Every call to bson.NewObjectId() will also call syscall.Getpid() to obtain the process id. This PR caches the id and increases the performance:

Before:
PASS: bson_test.go:1828: S.BenchmarkNewObjectId 10000000  228 ns/op

After:
PASS: bson_test.go:1828: S.BenchmarkNewObjectId 20000000 76.0 ns/op

AFAIK go does not support process forking, the process id should therefore always be the same.

I noticed this while working on a system that writes a lot of data to the database and generates the ObjectId on the client. This PR significantly increases the performance of the said system.

[niemeyer]

Thanks!

[256dpi]

Thanks! How long will it take until this change is merged into v2 (stable)?