Two threading tweaks [JIRA: RIAK-2336]

[matthewvon]

1. should create synchronous start-up for thread pool threads like recently done in throttle.cc. The change is not relevant to normal execution. It simply makes unit tests which construct and deconstruct quickly more reliable.
2. consider using first thread in a pool as replacement for semaphore thread. The semaphore thread is to protect against a race condition where no thread is available as new task arrives, but by the time the new task is on the queue all threads are back to waiting and will never see the task until a second task gets added. Using a conventional mutex and signal only with the first thread will prevent this case AND not heavily burden the system. (Known bad design had one mutex / condition variable for all threads. This was a performance killer.) ... of course performance impact needs to be tested.

[matthewvon]

Item two is complete: branch mv-no-semaphore. Checked into develop July 12, 2016.