table.optimize() should delete junk rows from *_fts_docsize

[simonw]

The second challenge here is cleaning up all of those junk rows in existing *_fts_docsize tables. Doing that just to the demo database from https://github-to-sqlite.dogsheep.net/github.db dropped its size from 22MB to 16MB! Here's the SQL:

DELETE FROM [licenses_fts_docsize] WHERE id NOT IN (
  SELECT rowid FROM [licenses_fts]);

I can do that as part of the existing table.optimize() method, which optimizes FTS tables.
Originally posted by @simonw in #149 (comment)

[simonw]

Writing a test for this will be a tiny bit tricky. I think I'll use a test that replicates the bug in #149.

[simonw]

FTS4 uses a different column name here: https://datasette-sqlite-fts4.datasette.io/24ways-fts4/articles_fts_docsize

CREATE TABLE 'articles_fts_docsize'(docid INTEGER PRIMARY KEY, size BLOB);

[simonw]

I've reverted this change again, because it turns out using the rebuild FTS mechanism is a better way of repairing this issue - see #155.