lib: mgmtd: session create and destroy both short-circuit

lib: mgmtd: session create and destroy both short-circuit

For creation this is the first thing done so short-circuit just means inline
sync response. However, for destroy there could be commands in-flight, these
will be discarded when they match no session, and the state cleaned up
immediately when the message short-circuits.

Signed-off-by: Christian Hopps <chopps@labn.net>