First two commits are #78539.
Closes #19749 (though this is still a draft, up for discussion)

This commit adds support for an "all versions scan" style from SQL down
to KV. It permits using the `AS OF SYSTEM TIME ALL` modifier to instruct
the system to return all MVCC versions of the scanned data, rather than
just the most recent under the read timestamp.

Release note: add AS OF SYSTEM TIME ALL, a modifier that gives all MVCC
versions on disk for the scanned data.

This is still a work in progress. Here is a checklist of things that we'll need to do before we can consider merging:

• scanner: support reverse scans
• scanner: more exhaustive tests of scans, for cases with intents or newer versions we'd like to not see, etc
• sql: plumb all versions flag into lookup join, index join, zigzag join, inverted joiner
• sql: teach the optimizer that all versions scans don't uphold uniqueness. Is this possible? It's important - if there's any place where we assume uniqueness, we need to disable that because we could see more than 1 MVCC version for a single key.
• sql: add more tests, especially for the cases that can come up from the above point about lost uniqueness.