MINOR: cli/wait: add a condition to wait on a server to become unused

MINOR: cli/wait: add a condition to wait on a server to become unused

The "wait" command now supports a condition, "srv-unused", which waits
for the designated server to become totally unused, indicating that it
is removable. Upon each wakeup it calls srv_check_for_deletion() to
verify if conditions are met, if not if it's recoverable, or if it's
not recoverable, and proceeds according to this, never waiting for a
final decision longer than the configured delay.

The purpose is to make it possible to remove servers from the CLI after
waiting for their sessions to be terminated:

  $ socat -t5 /path/to/socket - <<< "
        disable server px/srv1
        shutdown sessions server px/srv1
        wait 2s srv-unused px/srv1
        del server px/srv1"

Or even wait for connections to terminate themselves:

  $ socat -t70 /path/to/socket - <<< "
        disable server px/srv1
        wait 1m srv-unused px/srv1
        del server px/srv1"