SERVER-39106 fixed a problem about lock acquisition: When ticket acquisition times out due to _maxLockTimeout, it should throw rather than fail silently with LOCK_TIMEOUT result.

However, the contract of lock acquisition is not very clear right now:

1. If the lock acquisition times out due to _maxLockTimeout, it should throw.
2. If the lock acquisition times out due to deadline specified by callers, it should silently return LOCK_TIMEOUT

This ticket is to try to simplify the contract so that it always throw an exception.