Extract "last updated" info of aggregates from sql

See #2560

git-svn-id: http://svn.osgeo.org/postgis/trunk@12244 b70326c6-7e19-0410-871a-916f4a2858ee
Use last Changed/Availability in a multi-change comment

See #2560

git-svn-id: http://svn.osgeo.org/postgis/trunk@12245 b70326c6-7e19-0410-871a-916f4a2858ee
Print warning on missing last updated info in aggregates

git-svn-id: http://svn.osgeo.org/postgis/trunk@12246 b70326c6-7e19-0410-871a-916f4a2858ee