Pipe: wait for Pipes to finish running, instead of just stop()ing them

Add a stop_and_wait() function that, in addition to closing the Pipe and killing
its socket, waits for any fast_dispatch call which is in-progress. Use this in
several parts of the Pipe and SimpleMessenger code where appropriate.

This fixes several races with fast_dispatch and other avenues; here are two:
1) It could be that we grab the lock while the existing pipe is fast_dispatching
and then proceed to dispatch messages ourself, beating it. Instead, wait for
the other pipe. Add a "reader_dispatching" member which tells bus this is
happening, and when re-locking, signal the cond if we're shutting down.

2) It could be that a normally-dispatched Message in the OSD triggers a
mark_down() on the Connection and then clears out the Session
(Connection::priv) pointer, causing a racing fast_dispatch()'ed function to
assert out in the OSD because it requires a valid Session.

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