The main objective is prevent deletion of objects that can leave
unreachable orphan objects which we are unable to correctly track.
Additionally, this commit includes:
1. Properly implement safe cascade deletion of all participating
tables on soft deletion in the objects-gc task;
2. Make the file thumbnail related tables also participate in the
touch/refcount mechanism applyign to the same safety checks;
3. Add helper for db query lazy iteration using PostgreSQL support
for server side cursors;
4. Fix efficiency issues on gc related task using server side
cursors instead of custom chunked iteration for processing data.
The problem resided when a large chunk of rows that has identical
value on the deleted_at column and the chunk size is small (the
default); when the custom chunked iteration only reads a first N
items and skip the rest of the set to the next run.
This has caused many objects to remain pending to be eliminated,
taking up space for longer than expected. The server side cursor
based iteration does not has this problem and iterates correctly
over all objects.
5. Fix refcount issues on font variant deletion RPC methods
