Fix compute_instance migration bug.

[paddycarver]

compute_instance's StateVersion was set to 2. Then we released a
migration to v3, but never updated the StateVersion to 3, meaning the
migration was never run. When we added the migration for disks, we
bumped to 4, bypassing 3 altogher. In theory, this is fine, and is
expected; after all, some people may have state in version 0 and need to
upgrade all the way to 4, so our schema migration function is supposed
to support this.

Unfortunately, for migrations to v2, v3, and v4 of our schema, the
migration returned after each migration, instead of falling through.
This meant that (in this case), version 2 would see it needs to be
version 4, run the state migration to version 3, then return, setting
its StateVersion to 4, which means the migration from 3->4 got skipped
entirely.

This PR bumps the version to 5, and adds a migration from 4->5 such that
if there are still disks in state after 4, re-run 4. This will fix
things for people that upgraded to 1.0.0 and had their StateVersion
updated without the migration running.

I also updated the tests @danawillow wrote to start from state version 2
instead of state version 3, as the state would never be in version 3.

I also duplicated those tests, but started them from state version 4
(assuming the migration hadn't run) and verifying that the migration
from 4->5 would correct that.

Fixes #509.

[ghost]

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