[TECH DEBT]: Inefficient use of TableMigrationStatusRefresher.index()

[asnare]

Problem statement

The TableMigrationStatusRefresher.index() method is used to load the migration state of all tables and views in the inventory. The fast-path is that this loads the migration state from the inventory table (select * from migration_status).

We have a lot of code-paths that invoke this function to obtain the index, especially in the migration-related code, not realising that it's relatively expensive. One particular case is: TablesMigrator.get_remaining_tables() which loads this index (and discards it) within a loop over every table in the inventory.

Proposed Solution

At the very least TablesMigrator.get_remaining_tables() should be refactored to only build the index once.

I haven't some a full survey of usage, but there are probably many other places where refactoring is needed to avoid redundant loading of the index.