Upgrade fails when using database migration without restart

Issue Summary

Async database upgrade tasks don't run unless the server is restarted. Changing Bitbucket database using the database migration tool without doing a restart will mean that these upgrade tasks don't run.

Steps to Reproduce

1. Use database migration to change to a different database
2. Don't restart the upgraded server

Expected Results

Entire database should be migrated to the new database, including any async tasks. (For example the idx_bb_build_status_commit_id index on bb_build_status should be present.)

Actual Results

Async database upgrade tasks are not run.

Workaround

Restart the server