Implement data format changes as outlined in the technical design doc.  This work:

• adds (and potentially) rearranges the bits in the "additional descriptor byte" in cell.h as per the design doc
• change functions in cell.i to set or check the new bits, pack or unpack the implied new timestamp (there are now two durable timestamps).  Some of the inline functions may pass in or return new timestamps.

At the moment, we don't have to "bubble up" the changes much higher than that, as that will be handled in other tickets.  This work, and the equivalent work in WT-5669 for 4.2, gives us the bare minimum to start doing downgrade testing as in WT-5670.