Merge 'mvcc: add some plumbing for index support' from Jussi Saurio

Adds `index_rows` which is a map of maps (one `SkipMap<SortableIndexKey,
Vec<RowVersion>>` per index), and adds `mvcc_cursor_type` to
`MvccLazyCursor`. Some cursor operations are refactored to work on both
indexes and tables.
Nothing basically actually works with indexes yet, but trying to keep
PRs manageably sized.

Closes #4027

Author: jussisaurio