Managed DROP TABLE

[shlomi-noach]

Tracking issue: #6689

Implementing a TableGC (garbage collector) component as part of TabletServer, which periodically checks for tables in various table-drop states, transitions the tables between those states, purges data from the tables, or drops them altogether.

At this time this PR is not concerned with user facing DROP statements.

[shlomi-noach]

This PR is now ready for review. Still TODO:

• endtoend tests
• accompanying documentation

[shlomi-noach]

Please note this PR requires, and is based on, #6668 . It is best if we first merge #6668. Reviewing this PR will become easier.

[shlomi-noach]

This PR implements most of the DROP TABLE logic in #6689 : it runs the table lifecycle behind the scenes, down to dropping the table. The only part it does not implement is the user facing DROP IN '24h' TABLE my_table syntax. This will be taken care of by a followup PR.

This PR is enough to run table garbage collection, if only the user/app know how to RENAME their table into the correct magic form. That's easy to do if you use the mechanisms go/vt/schema. And this is exactly what #6547 (or one of its followups) will do at the end of migration, for the artifact tables.

[shlomi-noach]

endtoend tests now added

[shlomi-noach]

This PR is in good shape the way I see it. It works through a table's lifecycle to eventually drop it; it only works on tables with "magic" names like _vt_HOLD_6ace8bcef73211ea87e9f875a4d24e90_20200915120410.

It does not offer a DROP TABLE syntax -- that's for a different PR.

So, this PR only introduces a new behavior for very special tables, which will be immediately put to use by #6547 .

[shlomi-noach]

I'm ready to have this PR reviewed and merged.

[deepthi]

Looks fine to me.
We can merge after the conflicts are resolved and tests pass.

[shlomi-noach]

conflicts resolved, tests pass. Per review, I'm now merging (!)