maxPurgeableTimestamp should traverse tables in order of minTimestamp

In maxPurgeableTimestamp, we iterate over the bloom filters of each overlapping SSTable. Of the bloom filter hits, we take the SSTable with the lowest minTimestamp. If we kept the SSTables in sorted order of minTimestamp, then we could short-circuit the operation at the first bloom filter hit, reducing cache pressure (or worse, I/O) and CPU time.

I've written (but not yet benchmarked) some code to demonstrate this possibility.