This extracts information which was there in row_cache.md, but is
relevant to MVCC in general.
It also makes adaptations and reflects the upcoming changes in this
series related to switching to the new mutation_partition_v2 model:
- continuity in evictable snapshots can now overlap. This is needed
to represent range tombstone information, which is linked to
continuity information.
- description of range tombstone representation was added
