Fix deadlock clearing data on migration failure

[SessionHero01]

This PR is to fix this issue on migration error screen: tapping on "clear data" does nothing. See QA-1477.

There are two issues here:

1. As all database access is blocked waiting on error screen, and the config system was holding a lock while accessing the db, the "clear data" unfortunately will also need access to the config system. The lock then become a deadlock. The solution here is to remove the lock while creating the config from db.
2. The use of Default dispatcher might not be ideal as all default dispatcher threads are blocked waiting on db. Here for clearing the data, we'll need to use the unconstrained IO dispatcher.