MINOR: checks: add on-marked-up option

MINOR: checks: add on-marked-up option

This implements the feature discussed in the earlier thread of killing
connections on backup servers when a non-backup server comes back up. For
example, you can use this to route to a mysql master & slave and ensure
clients don't stay on the slave after the master goes from down->up. I've done
some minimal testing and it seems to work.

[WT: added session flag & doc, moved the killing after logging the server UP,
 and ensured that the new server is really usable]