Change WT data format to avoid writing stable timestamps

Writing pairs of timestamps for key/value pairs will potentially grow data sizes (it's at least one byte per timestamp, and likely two or more).

Investigate adding a cell bit that flags the "WT_TS_NONE - WT_TS_MAX" pair, that is, the timestamp pair that indicates global stability.

It's not known if this is a possible change, we may investigate it and decide it's not possible.