Use descriptions for compaction

[frankmcsherry]

Batches come with "descriptions", which contain lower and upper bounds on the timestamps of updates contained within. Specifically, the times within are those greater or equal to some element of lower and not greater or equal to any element of upper.

It seems like we should be able to use these bounds to inform advance compaction, in which we consider advancing all times in a batch by a provided frontier. The frontier's interaction with lower and upper seems like it constrains what happens to the times in the batch, and it could tell us about whether we should expect the frontier to collapse all times down, or collapse no times at all, or what.

This information could let us advance more aggressively when appropriate, and to put off compaction when we can see that the involved batch is unlikely to be much improved.