409 retry can result in invalid remote ancestor ID

This is an edge case, but was reported in a CBSE.  If a backup of a server bucket is made, but replication continues, and then later the bucket is restored this invalidates the remote ancestors in the local database and causes a lot of conflicts that end up in a situation where CBL thinks that the parent rev ID is the same as the current rev ID.  Half a day of research went into this so it's not easily summarized here but basically the checkpoint is rewound, causing a replication restart in which for some reason the remote ancestor persists so when the pusher goes to send certain revisions the sequence goes something like this:

1. Send doc gen 2 with no parent
2. 409 received
3. Lookup remote ancestor and retry with remote ancestor (this fails because the remote ancestor is the same gen 2 revision ID)