Clarify logging in lockutils

The old logging statements in lockutils made for some confusing log
entries where it would appear a given lock was acquired multiple
times by different threads at the same time.  See referenced bug
for details.

In order to alleviate that confusion, this change does a few things:
1) Adds an explicit "acquired" message inside the lock so it is
   clear when the lock was actually acquired.
2) Moves the release message inside the semaphore so there's no
   chance of it being logged out of order.
3) Removes the "Got semaphore" message and splits it into two
   separate messages depending on whether the semaphore was found
   in the weakref dictionary.  Making it clear which code path
   was followed should help with future debugging.

Change-Id: I0fbb473c60d48c9704597d9e3634402857861a66
Closes-Bug: 1367941
Merge "Clarify logging in lockutils"