osd: handle missing/degraded in op thread

osd: handle missing/degraded in op thread

The _handle_op() method (and friends) are called when an op is initially
queued and when it is requeued.  In the requeue case we have to be more
careful because the caller may be in the middle of doing all sorts of
random stuff.  That means we need to limit ourselves to queueing or
discarding the op, and refrain from doing anything else with dangerous
side effects.

This fixes a crash like

osd/ReplicatedPG.cc: In function 'void ReplicatedPG::recover_primary_got(hobject_t, eversion_t)', in thread '7f21d0189700'
osd/ReplicatedPG.cc: 4109: FAILED assert(missing.num_missing() == 0)
 ceph version 0.37-105-gc2069eb (commit:c2069eb1e562ba7d753c9b5ce5c904f4f5ef6abe)
 1: (ceph::__ceph_assert_fail(char const*, char const*, int, char const*)+0x76) [0x8ab95a]
 2: (ReplicatedPG::recover_primary_got(hobject_t, eversion_t)+0x62e) [0x767eea]
 3: (ReplicatedPG::sub_op_push(MOSDSubOp*)+0x2b79) [0x76abeb]
 4: (ReplicatedPG::do_sub_op(MOSDSubOp*)+0x1ab) [0x74761b]
 5: (OSD::dequeue_op(PG*)+0x47d) [0x820ac3]
 6: (OSD::OpWQ::_process(PG*)+0x27) [0x82cc8b]

due to an object being pushed to a replica before it is activated.

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