Fix crashes, print some stats

Notably:
- fix the `delete op` inside lambda callback crash (it frees the lambda itself
  which results in use-after-free with g++)
- fix stop_client() reenterability
- fix a bug in the blockstore layer which resulted in always returning version=0
  for zero-length reads
- change error codes for blockstore_stabilize

osd_flush.cpp

@@ -254,6 +254,7 @@ resume_4:
         auto st_it = pg->degraded_objects.find(recovery_state.oid);
         st = st_it->second;
         pg->degraded_objects.erase(st_it);
+        degraded_objects--;
     }
     st->object_count--;
     if (st->state == OBJ_DEGRADED)