SQUASH: msgr: close a race in DelayedDelivery::wait_for_flush()

If we managed to grab the delay_lock while the delay_thread is flushing
its last message, we will see that flush_count==0 and return right away,
despite the thread not being ready to stop/join. Close that hole.

Signed-off-by: Greg Farnum <greg@inktank.com>
SQUASH msgr: steal the delay_thread instead of its data structures

This means we don't have to wait for it to finish any fast_dispatch work
it's presently doing before we move on.

Signed-off-by: Greg Farnum <greg@inktank.com>