Sync produces conflict copy of gpkg without a reason

[wonder-sk]

In #52 the newly added test test_missing_basefile_pull triggers an issue - instead of doing a rebase, a new conflicting copy gets created. We should investigate why that is happening...

[wonder-sk]

Further investigation shows that with missing basefile the client is unable to do rebase. Even when the newest basefile is requested, we are missing the "old" basefile. The solution would be to either: A. download newest basefile and subtract diffs from it (if they are available), or B. download older basefile

[wonder-sk]

This looks like an instance of MerginMaps/qgis-plugin#246 where the schema changes and leads to an error and subsequent removal of basefile.