osd/mon: subscribe (onetime) to pg creations on connect

osd/mon: subscribe (onetime) to pg creations on connect

Ask the monitor for pending pg creations each time we connect.

Normally, this is a freebie check.  If there are pending creations, though,
it ensures that the OSD finds out about them even if the original lame
broadcast didn't reach it.  Specifically:

 - osd is hunting for a monitor, but isn't yet connected
 - new pgs are created
 - send_pg_creates() sends out create messages, but osd does get it
 - osd finally connects to a mon

Fixes: #2151 (tho the bug description is bad)
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>