Fix compaction bug: flush compaction queue if it's too old.

The compaction queue needs to be flushed if either it's grown too
large or it's been too long since the last flush.  That second
condition wasn't implemented properly because the timestamp that
tracks the time at which we last flushed was never set and always
remained 0.  This would only impact very low-traffic TSDs that
never manage to exceed the threshold on the size of the queue.

Thanks to Brent Evans for spotting and reporting this bug.