Remove note
Migration to create revision tables

Transforms separate revision tables into a single table
with a `revision` column (and adds a `revision` column
onto the content table.
Create revisions from the `content_revisions` table

This means that the per-revision tables are now only needed during the
lifetime of the revision and can be safely deleted afterwards.
Add a revision clean-up method

Just need to integrate this into the full publish process (after the
new revision has been made live)
Get name of content base table from mapper
Simplify test & ensure revision list is sorted
Speed up visibility test
Remove un-needed fiber-pool dependency
Use xapian-ruby in place of xapian-full
Invoke revision cleanup task as last step in publish
Better name for conflict test

Also make sure the tests are running with a consistent state by
resetting the site.