Improved DeletionTime serialization

DeletionTime.markedForDeleteAt is a long useconds since Unix Epoch. But I noticed that with 7 bytes we can already encode ~2284 years. We can either shed the 8th byte, for reduced IO and disk, or can encode some sentinel values as flags there. blerer suggested starting with DeletionTime.LIVE.

That would mean reading and writing 1 byte instead of 12 (8 mfda long + 4 ldts int). Yes we already avoid serializing DeletionTime (DT) in sstables at row level entirely but not at partition level and it is also serialized at index, metadata, etc.