Add a last_update column for use in the scavenger
In this merge request a column is added to the obm_buildings table, so the scavenger can iterate through the table and update each building that has been unchanged for the longest time. This way we can remove buildings that exist in our obm_buildings database but do not in the osm_replication database (See for example ID -2270769 in the south of Budapest, that is some land area that has been mistakenly classified as building).
Also, an index on the last_update column is added.
Edited by Laurens Oostwegel