director: Added "up" vs "down" states and doveadm director up/down commands.

director: Added "up" vs "down" states and doveadm director up/down commands.
These commands are intended to be used by automated watchdogs that detect if
backends are up or down. This way the vhost count doesn't get forgotten
after server goes down. It also means that admin can manually take down a
server by setting its vhost count to 0 without the watchdog automatically
bringing it back up.