PG: drop messages from down peers

This overlaps with the existing old_peering_msg() mechanism
except in one case: pulls from a replica not in the acting
set.  If such a replica gets marked down, we may resend
pulls to another replica without causing a new interval
to start.  If we recieved, but didn't process, a push in
response to such a pull prior to processing the map marking
the peer down, we might process the push after having reset
the pull state for a different pull operation.  We can
avoid this by discarding ops from down peers.

Signed-off-by: Samuel Just <sam.just@inktank.com>