MB-12540: Refactor the way we track snapshot start/end seqnos

For the 3.0 release I needed to add snapshot sequence number
quickly and in doing so implemented an approach that was not
very good and prone to errors. The correct approach is to track
sequence numbers on disk and also to track them for each checkpoint
in the checkpoint manager. This change moves us to the better
approach.

Change-Id: Ifbdd6c87b06767ff8c6504c1058ce11195fa2428