Make numeric user_id checker start at @0, and don't ratelimit on checking

[anoadragon453]

Fixes #5225

This PR:

• Changes find_next_generated_user_id_localpart to start at @0 instead of @1, so we don't tell people it's ok to register @0 and then they proceed to try every number up the entire table
• I've removed the rate limiting check from register_with_store and moved it up and out, also eliminating the need to have an explicit check for application service registration.

[erikjohnston]

Just some niggles. Can you also put a limit on the autogenerate user ID loop please? That way if we do hit the bug again we will only loop a few times before giving up, rather than taking out the servuce.

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