clear_all removes initial message

[imbolc]

After SELECT mq_clear_all() a couple of tests fail with
Some("Key ( after_message_id)=(00000000-0000-0000-0000-000000000000) is not present in table \"mq_msgs\".")

[Diggsey]

Ah that;s a clear bug. We should leave the "nil" message untouched.

[imbolc]

Would it make sense to move it to Rust as we discussed? With sql functions users have to remember to run migrations when they upgrade.

[Diggsey]

I think this is a special case as it's convenient to be able to reset the database directly in postgres.