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

blockstore.h

@@ -93,7 +93,7 @@ Input:
 - buf = pre-allocated obj_ver_id array <len> units long
 
 Output:
-- retval = 0 or negative error number (-EINVAL or -EBUSY if not synced)
+- retval = 0 or negative error number (-EINVAL, -ENOENT if no such version or -EBUSY if not synced)
 
 ## BS_OP_SYNC_STAB_ALL