[1094540] Agents get permanently backfilled if backend database goes offline for a moment

Don't unset the backfill flag in the ping logic, just make
sure the agent knows the server thinks its down, and let it
resolve the situation by sending a full avail report, which when
processed will unset the backfill flag.
Merge pull request #38 from rhq-project/jshaughn/1094540

[1094540] Agents get permanently backfilled if backend database goes off...