Recording of migrations with unmatched assumptions.

[michael-simons]

We should reconsider the fact that we don't record migrations with unmatched assumptions. Right now, they are skipped as a whole in the chain and are not recorded. That leads to issues:

Skipping a migration based on a version (version >= x, migrations no longer applied)

Given 3 migrations on version < x
Chain will be 1,2,3
Database will be upgrade to x
Chain will be 1,3, validation will fail due to unexpected migration (3 in this case, already being recorded)

It would have been prevented by having an "empty" migration for version x.

Migrations suddenly applied

Same, but chains switched.

cc @meistermeier @ali-ince