When moving the finding of linked places to the precomputation stage,
it was also moved before the statement where the linked_place_id was
removed from the linkee. The result was that the current linkee was
excluded when looking for a linked place on updates because it was
still linked to the boundary to be updated.
Fixed by allowing to either keep the linkage or change to an unlinked
place.
Before updating an admin boundary we need to make sure that any
artificially generated 'linked_place' entry is removed from the
extratags column. This ensures that the place designation does
not linger when a linked place disappears and that it is updated
when the linking changes.
