msgr: discard the local_pipe's queue on shutdown.

msgr: discard the local_pipe's queue on shutdown.

To facilitate this, we do two things:
1) actually identify the number of special code values we pass around
2) use that to prevent trying to put() those non-pointer values in
Pipe::discard_queue().
Then we just call local_pipe.discard_queue() in wait() like happens
(indirectly, via reaping) with all the normal Pipes in rank_pipe.

But this does make me think that we may be approaching the point
where it's appropriate to create a subclass LocalPipe (against a
RemotePipe like our current Pipe implementation is mostly intended
to be).

Should fix #2086.

Signed-off-by: Greg Farnum <gregory.farnum@dreamhost.com>
Reviewed-by: Sage Weil <sage@newdream.net>