apply, flush and purge take the try lock and fail with ErrPending, which now
names 'tomswall revert' as the recovery after a failed automatic revert. Each
try gets an ID passed to the timer's 'revert --id', so a stale timer cannot
revert a newer try. Adds tests for Revert success/failure/stale ID and for
restoring a present table at the engine level.
unkin-agent