SERVER-14668 Lock manager's LockResource should point to the LockHead

This is so we don't have to look up the lock head from the resource every
time.

Also moves the main locking functionality in the LockerImpl to a private
method, which will be used in more than one place after MMAP V1 flush lock
yielding is added.
SERVER-15525 Split sharding map/reduce tests
SERVER-15178 Setting LTC_NO_ASM macro
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
Merge remote-tracking branch 'upstream/master'