osd: queue pg removal under pg's epoch

osd: queue pg removal under pg's epoch

The PG may be doing work relative to a different epoch than what the osd
has.  Make sure the PG removal message is queued under that epoch to avoid
confusing/crashing the recipient like so:

2012-02-10 23:26:35.691793 7f387281f700 osd.3 514 queue_pg_for_deletion: 0.0
osd/OSD.cc: In function 'void OSD::handle_pg_remove(OpRequest*)' thread 7f387281f700 time 2012-02-10 23:26:35.691820
osd/OSD.cc: 4860: FAILED assert(pg->get_primary() == m->get_source().num())

Signed-off-by: Sage Weil <sage@newdream.net>