- mds0 triggers shutdown by sending a shutdown_start to all nodes.  

- from here on out, all client requests are discarded (unless they are a file close?)

- each mds checks for outstanding inter-mds transations.  e.g imports, discoveries, etc.  once they're all done, send a shutdown_ready to mds0

- each mds successively disassembles its cache, flushing data to long-term storage, and sending inodeexpires, exporting imported dirs to parent (after they're clean + empty)

- when the cache is empty, send shutdown_done to mds0 and exit.

- mds0 exits when all mdss have finished.