factor out code that handles depositing of work items on the
  thread pool's request queue.
- when it looks as if a new worker thread needs to be created, give
  up our quantum first in the hope that this might at the last minute
  turn up more idle worker threads.
- add comment re: trimming pool size.

Merged to STABLE eventually; I may continue tinkering with this code
some more over the next day or two.