client: allow client to leave state intact on MDS when tearing down objects
When ganesha shuts down cleanly, it'll tear down all of its filehandle objects and release the files that it has open. This is problematic in a clustered configuration where we want the MDS to leave state intact while a host is being migrated.
We could just leak this state, but that means working with something like asan is more difficult.
Allow the client to tear down its state without communicating with the MDS. The MDS should just see the client as dropping offline, but the client should be able to release its state cleanly.