osd: tolerate unexpected messages on the heartbeat interface

We should note but not crash on unexpected messages.  Announce this awesome
new "capability" via a feature bit.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
osd: only share maps on hb connection of OSD_HBMSGS feature is set

Back in 1bc419a7affb056540ba8f9b332b6ff9380b37af we started sharing maps
with dead osds via the heartbeat connection, but old code will crash on an
unexpected message.  Only do this if the OSD_HBMSGS feature is present.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>