CollectionWriter refactoring

RUBY-676
max_write_batch_size accessor for performance tests

RUBY-676
bulk write exception result and emulation with old write operations

RUBY-676
bulk write test fix for emulation with old write commands

RUBY-676
Merge branch '1.x-stable-ruby-676' of github.com:gjmurakami-10gen/mongo-ruby-driver into gjmurakami-10gen-1.x-stable-ruby-676