3.0.0-beta.3 - AsyncQueue and lots more tests

Detailed changes:

• The queue driver now adopts the new AsyncQueue protocol and no longer has to go back and forth between futures.
• Adopters of this package are no longer forced to have all three of the Fluent SQL database driver stacks (SQLite, Postgres, MySQL) in their dependencies (although most modern versions of SwiftPM manage to avoid this problem already); the tests instead will only use whichever drivers were included at the time of the tests being built (defaults to none, except in CI).
• The StoredJobState.completed enum case has been removed, since we never actually used it. This incidentally improves the behavior of the DELETE query in the queue's clear() method.
• The tests are now more comprehensive.
• More Sendable warnings fixed.
• Made the database queries (even) more readable.