DB/Overpass: Bugfix compiling parent selectors: filter for set statements
DB/Overpass: handle set statements in parent relationships
DB/Overpass: merging regexps: bugfix (could not be convertable)
DB/Overpass: merging regexp: also handle parent_ conditions
Main loop: a couple of bugfixes

- initialize src before adding objects from request queue
- re-initialize combine_objects
- after finishing a source, set 'src' to None
- process additional objects regardless of pending_min_index
Main loop: debug.loop shows processing of main loop
Eval: bugfix compiling call for possible_values()
DB/Overpass: Bugfix object_members(): might not be in cache -> return nothing
DB/Overpass: objects_members(): only return parents in viewport