[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 we need a full avail report sent and defer
the unsetting to when we actually get a report.
Merge pull request #37 from rhq-project/jshaughn/1094540

Jshaughn/1094540 reviewed by mazz and loleary.  Approved for 3.1.x.