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_rollback.cpp

@@ -24,7 +24,7 @@ int blockstore_impl_t::dequeue_rollback(blockstore_op_t *op)
                 // FIXME Skip this object version
             }
         bad_op:
-            op->retval = -EINVAL;
+            op->retval = -ENOENT;
             FINISH_OP(op);
             return 1;
         }