Online DDL: remove legacy "stowaway table" logic

[shlomi-noach]

Description

The "stowaway table" was part of the previous vitess cut-over mechanism, described in https://vitess.io/blog/2022-04-06-online-ddl-vitess-cut-over/

However, the logic has changed in #11460. We no longer use a stowaway table in the cut-over mechanism. The stowaway table was the place where we would store our original table at cut-over phase. But because the cut-over was not atomic, we had to journal that changed. That was done with the stowaway_table column in _vt.schema_migrations. We would check this value to be able to rollback a halfway executed migration.

Anyway, now we use atomic cutover. We have a "sentry table" now. But this isn't merely a change of name. The sentry table is ephemeral. That is, it's a real table, but we don't care what happens to it. It's temporary in use. There's no situation where we have to resurrect it.

This PR cleans up all the stowaway table logic, which is unused now. To clarify how unused it is, the only scenario where anyone would still need this logic is if they had a failover during a cut-over phase, and at the same time chose to upgrade their vitess cluster. Which is unreasonable.

Related Issue(s)

• Online DDL: tracking issue #6926
• vitess Online DDL atomic cut-over #11460

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• If this is a change that users need to know about, please apply the release notes (needs details) label so that merging is blocked unless the summary release notes document is included.
• If a test is added or modified, there should be a documentation on top of the test to explain what the expected behavior is what the test does.

If a new flag is being introduced:

• Is it really necessary to add this flag?
• Flag names should be clear and intuitive (as far as possible)
• Help text should be descriptive.
• Flag names should use dashes (-) as word separators rather than underscores (_).

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow should be required, the maintainer team should be notified.

Bug fixes

• There should be at least one unit or end-to-end test.
• The Pull Request description should include a link to an issue that describes the bug.

Non-trivial changes

• There should be some code comments as to why things are implemented the way they are.

New/Existing features

• Should be documented, either by modifying the existing documentation or creating new documentation.
• New features should have a link to a feature request issue or an RFC that documents the use cases, corner cases and test cases.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• vtctl command output order should be stable and awk-able.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from VTop, if used there.

[mattlord]

Shouldn't we remove it from the schema here too? https://github.com/vitessio/vitess/blob/main/go/vt/sidecardb/schema/onlineddl/schema_migrations.sql#L61

[shlomi-noach]

Shouldn't we remove it from the schema here too?

Yes, indeed. Done.

[deepthi]

I assume no tests directly tested this internal detail.