base.join_and_indicate_progress() waits for the pool to complete its
work, printing indication dots to stdout once per second. If it takes
less than a second for the pool to complete, we won't print anything
(and we also won't hang for a second waiting for nothing to happen).

The previous implementation required the caller to store a running tally
of how many commands had been added to the pool; that requirement is now
dropped.

Unlike wait_and_printdots(), join_and_indicate_progress() *always*
prints to file. Don't call it if you don't want to print; use
WorkerPool.join() directly.