d=M_close() will always do the verbose clean shutdown thing, and the Deferred
won't fire (e.g. we won't move to state "Ss") until we've deallocated our
server resources (nameplates and mailboxes), and we've finished shutting down
our websocket connection. So integration tests should wait on the Deferred to
make sure everything has stopped moving and the reactor is clean.
CLI applications that are following the success path can use M_close() and
wait on the Deferred before terminating.
CLI applications that wind up on some error path can either use M_close(), or
just SIGINT and leave the server to GC stuff later.
GUI applications can use M_close() but ignore the Deferred, and assume that
the program will keep running long enough to get the deallocation messages
through.
GUI+DB applications can use M_close() and then stop recording state changes,
and if the program remains running long enough, everything will be
deallocated, but if it terminates too soon, the server will have to GC.
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