SERVER-14425 Cleanup LockerImpl

* Remove unused methods
* Make references to the lock manager to go directly through the instance
* Add comments
SERVER-14425 Performance optimization in LockManager for the unlock path
SERVER-14425 Make MMAP V1 code in LockerImpl conditionally compiled

This change cuts down on the locking work that other storage engines have
to do.
Merge remote-tracking branch 'root/master'
fix cursor cache
SERVER-13635: do not invalidate doc when using doc locking