[3.1.9 backport] Failure to perform on-demand import should result in not found/noRev

This was partially addressed in CBG-3697, but there are still cases where a failure to import the document isn't correctly sending a noRev message to clients, causing CBL to become stuck.

In the scenario where a rev is not found because the document has been mutated, we'll attempt to perform an on-demand import before checking for the revision. (and on main, potentially sending a replacementRev).  

If that on-demand import fails for any reason (even transient write error attempting to persist the imported doc), we should send a noRev message to the client.  The fact that it needed to be imported indicates that the document has been mutated since the revision the client received a changes message for.