SERVER-14668 expand support for yielding, add back in yield policy
SERVER-14668 Fix compile break due to a previous commit
Revert "SERVER-14668: Replace uses of DBWrite lock with DBLock"

This reverts commit da599844c97ef6d290c03e073a9bafe41c1a914e.

Conflicts:
	src/mongo/db/repl/repl_coordinator_external_state_impl.cpp
	src/mongo/dbtests/counttests.cpp
	src/mongo/dbtests/querytests.cpp