Locking logging tweaks and renewal fix

* Added new logging statements and reworded existing statements in
  ConsistentKeyLockTransaction and LocalLockMediator

* LocalLockMediator#unlock() now logs errors and returns false instead
  of generating assertion failures when called on an expired or
  nonexistent lock; call sites updated accordingly

* Replaced broken unit test in LockKeyColumnValueStoreTest named
  relockExtendsLocalExpiration with a new test method called
  repeatLockingDoesNotExtendExpiration (closes #264)
Made ByteBufferUtil#compare() thread-safe

ByteBufferUtil still has some methods besides compare() that look
thread-unsafe.  These might need review.
Fix indexing in ByteBufferUtil#compare()

In my last commit, I neglected to adjust arguments to
ByteBuffer#get(int index) by ByteBuffer#position().  This meant
compare() was broken for ByteBuffers whose position() was nonzero.