ctdbd: Replace ctdb->done_startup with ctdb->runstate

ctdbd: Replace ctdb->done_startup with ctdb->runstate

This allows states, including startup and shutdown states, to be
clearly tracked.  This doesn't include regular runtime "states", which
are handled by node flags.

Introduce new functions ctdb_set_runstate(), runstate_to_string() and
runstate_from_string().

Signed-off-by: Martin Schwenke <martin@meltin.net>
Pair-programmed-with: Amitay Isaacs <amitay@gmail.com>