msg/Pipe: inject delay in stop_and_wait

Inject a delay in stop_and_wait.  This will mostly affect the connection
race Pipe takeover code which currently calls stop_and_wait while holding
the msgr->lock.  This should make it easier for a racing fast_dispatch
method to get stuck on a call that (indirectly) needs the msgr lock.
See #9921.

Signed-off-by: Sage Weil <sage@redhat.com>