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_write.cpp

@@ -42,7 +42,7 @@ bool blockstore_impl_t::enqueue_write(blockstore_op_t *op)
     else if (op->version < version)
     {
         // Invalid version requested
-        op->retval = -EINVAL;
+        op->retval = -EEXIST;
         return false;
     }
     if (deleted && is_del)