This PR addresses a pg_deadlock error that would cause an update to fail if a `select getmvt` was queried during update. The deadlock occurred due to an ALTER TABLE DROP COLUMN operation on temporary columns ( `new_source_ids` and `old_source_ids`) in some linestring tables.
**Changes:**
* instead of adding/dropping the columns during update, they are now updated and set to NULL.
* Added new indexes on the source_ids columns to speed up the UPDATE.
* Will refresh `osm_park_polygon_dissolve_z4` concurrently in `park_polygon.refresh()` to avoid blocking.
Co-authored-by: Patrik Sylve <patrik.sylve@t-kartor.com>
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