#14 dbdown.lock auto expiration

[Steve Southwell]

Taken from a previous bug report, and reposted here as
a feature request:

"There needs to be a method of having the agents
come back live on their own. Right now, we can have a
cron script remove the dbdown.lock file at a certain
time, but in the case where we're doing a weekly .bi
truncation, or offline-backup or something archaic like
that, we would have to set that time with a safety
margin, which causes unnecessary outage. If the agents
can come back on their own, you save some time. "

Background: dbdown.lock is a mechanism of letting FFW
know that certain databases are down on purpose, and
rather than checking/reconnecting with each hit, we
should let them be, and just display the contents of
dbdown.lock to the user. This would be something like
"We are temporarily doing maintenance, and expect the
site to be back up soon" or something to that effect.

The difficulties with making dbdown functionality reset
itself when the databases come back up are several:

1. Which databases coming up determine that dbdown is
no longer in effect? The default set?
2. What if you placed the dbset.down file before
taking down your databases, and FFW immediately
determined that the databases were in-fact running?
Should there be a minimum amount of time before
self-righting? If so, what is it, and how can it be
specified?