Encode availability and last change in raster aggregates

See #2560

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12238 b70326c6-7e19-0410-871a-916f4a2858ee
Encode availability of topology aggregates

See #2560

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12239 b70326c6-7e19-0410-871a-916f4a2858ee
Soft upgrade: avoid drop/recreate of aggregates that hadn't changed

Closes #2560

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12240 b70326c6-7e19-0410-871a-916f4a2858ee