(SERVER-150) Put a 'retry' poison pill in the flushed pool

When we replace the old pool with the new pool during a 'flush'
request, we need to have a way to handle the possible race
condition where some code has a reference to the old pool
and tries to borrow an instance from it after it has already
been flushed.  This commit is the first step towards that;
it puts a RetryPoisonPill into the old queue.