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_stable.cpp

@@ -55,7 +55,7 @@ int blockstore_impl_t::dequeue_stable(blockstore_op_t *op)
             if (clean_it == clean_db.end() || clean_it->second.version < v->version)
             {
                 // No such object version
-                op->retval = -EINVAL;
+                op->retval = -ENOENT;
                 FINISH_OP(op);
                 return 1;
             }