Fix duplicate entries in ps_sync_state

[simolus3]

The priority column in ps_sync_state should have been a primary key to prevent duplicates (we're already using INSERT OR REPLACE when inserting).

This bumps the schema version and re-creates the table when migrating, only keeping the most recent item in each priority from the old ps_sync_state table.
I've also added tests asserting that:

1. Inserting data into the old schema and then migrating only keeps the relevant rows.
2. Calling sync_local multiple times with a VFS mocking the current time inserts or updates rows in ps_sync_state as expected, and doesn't cause duplicate entries.