Make DTCS split SSTables to perfectly fit time windows

The time windows that DTCS uses are what the strategy tries to align SSTables to, in order to get the right structure, for best performance. I added the ticket CASSANDRA-8360, taking SSTables one step closer to aligning with these windows in a 1:1 manner.

The idea in this ticket is to perfectly align SSTables with the DTCS time windows, by splitting SSTables that cross window borders. This can lead to certain benefits, perhaps mostly in consistency and predictability terms, where it will be very well defined where every value is stored that is old enough to have stabilized.

Read queries can be aligned with windows in order to guarantee a single disk seek (although then the client needs to know the right window placements). Basically, SSTables can be made to align perfectly on day borders, for example. Right now, there would be an SSTable that almost represents a day, but not perfectly. So some data is still in another SSTable.

It could also be a useful property for tombstone expiration and repairs.

Practically all splits would happen only in the latest time windows with the newest and smallest SSTables. After those are split, DTCS would never compact SSTables across window borders. I have a hard time seeing when this could cause an expensive operation except for when switching from another compaction strategy (or even from current DTCS), and after a major compaction. In fact major compaction for DTCS should put data perfectly in windows rather than everything in one SSTable.