run unordered bulk operations in one batch per write-type

RUBY-676
Headroom defined for max sizes with correct handling

RUBY-676
sort_by_first_sym for old Rubies, suggestions from code review

RUBY-676
platform dependencies for max size exception tests

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