fix(pool): make sure there are no connection in the pool after close()

fix(pool): make sure there are no connection in the pool after close()

The case has been reported in #784. While not easy to reproduce, it
seems that it might be caused by the pool being closed while a worker is
still trying to create a connection, which will be put in the _pool
state after supposedly no other operation should have been performed.

Stop the workers and then empty the pool only after they have stopped to
run.

Also refactor the cleanup of the pool and waiting queue, moving them
to close(). There is no reason why a method called "stop workers" should
empty them, and there is no other code path that use such feature.

Close #784.