There are cases where we want to conditionally take a lock, but still
would like to use an RAII type to make sure we don't leak the lock.
This was previously impossible to do with `MutexLocker` due to it's
design. This commit tweaks the design to allow the object to be
initialized to an "empty" state without a lock associated, so it does
nothing, and then later a lock can be "attached" to the locker.
I realized that the get_lock() API's where also unused, and would no
longer make sense for empty locks, so they were removed.