osd/OSDMonitor: make 'osd pool rmsnap ...' not racy/crashy

Ensure that the snap does in fact exist before we try to remove it.  This
avoids a crash where a we get two dup rmsnap requests (due to thrashing, or
a reconnect, or something), the committed (p) value does have the snap, but
the uncommitted (pp) does not.  This fails the old test such that we try
to remove it from pp again, and assert.

Restructure the flow so that it is easier to distinguish the committed
short return from the uncommitted return (which must still wait for the
commit).

     0> 2013-07-16 14:21:27.189060 7fdf301e9700 -1 osd/osd_types.cc: In function 'void pg_pool_t::remove_snap(snapid_t)' thread 7fdf301e9700 time 2013-07-16 14:21:27.187095
osd/osd_types.cc: 662: FAILED assert(snaps.count(s))

 ceph version 0.66-602-gcd39d8a (cd39d8a6727d81b889869e98f5869e4227b50720)
 1: (pg_pool_t::remove_snap(snapid_t)+0x6d) [0x7ad6dd]
 2: (OSDMonitor::prepare_command(MMonCommand*)+0x6407) [0x5c1517]
 3: (OSDMonitor::prepare_update(PaxosServiceMessage*)+0x1fb) [0x5c41ab]
 4: (PaxosService::dispatch(PaxosServiceMessage*)+0x937) [0x598c87]
 5: (Monitor::handle_command(MMonCommand*)+0xe56) [0x56ec36]
 6: (Monitor::_ms_dispatch(Message*)+0xd1d) [0x5719ad]
 7: (Monitor::handle_forward(MForward*)+0x821) [0x572831]
 8: (Monitor::_ms_dispatch(Message*)+0xe44) [0x571ad4]
 9: (Monitor::ms_dispatch(Message*)+0x32) [0x588c52]
 10: (DispatchQueue::entry()+0x549) [0x7cf1d9]
 11: (DispatchQueue::DispatchThread::entry()+0xd) [0x7060fd]
 12: (()+0x7e9a) [0x7fdf35165e9a]
 13: (clone()+0x6d) [0x7fdf334fcccd]
 NOTE: a copy of the executable, or `objdump -rdS <executable>` is needed to interpret this.

Signed-off-by: Sage Weil <sage@inktank.com>