Option to disable min timestamp check for TTL compaction enhancements

As part of CASSANDRA-5228 we added an enhancement to unlink SSTables for TTL use cases if certain checks are met. One of those checks is that for an SSTable to be deleted the maximum timestamp must not be less than the minimum timestamp for all other SSTables. This makes sense for use cases where GC grace is >= the TTL, or use case where deletes are performed by the application.

For use cases where GC grace is less than the TTL, and where deletes are only performed via TTL then these checks result in SSTables that could be safely deleted sticking around for some time. In practice the TTL related enhancements kick in very infrequently and most SSTables go through the normal compaction process.

What I propose is a CF level setting that disables the check, so that an SSTable can be unlinked once time() >= max TTL.