mq: extract checklocalchanges from `mq.queue` The core part of `checklocalchanges` is now mq independent. We can extract it in a standalone function to help the extraction of `strip` as discussed in issue3824. A `checklocalchanges` function stay in `mq.queue` with the part related to "refresh first" messages.

mq: extract checksubstate from the queue class This function does not need any of the the `mq.queue` method or attributes. It is indirectly used by the `strip` command. We are trying to extract this command in a standalone extension as discussed in issue issue3824.