add migration for SceneGroupID to char(36) plus add an index. This

should actually speed up deletes substantially, especially for large
regions, as this was a table scan before.

OpenSim/Data/MySQL/Resources/011_RegionStore.sql

@@ -0,0 +1,9 @@
+BEGIN;
+
+ALTER TABLE prims change SceneGroupID SceneGroupIDold varchar(255);
+ALTER TABLE prims add SceneGroupID char(36);
+UPDATE prims set SceneGroupID = SceneGroupIDold;
+ALTER TABLE prims drop SceneGroupIDold;
+ALTER TABLE prims add index prims_scenegroupid(SceneGroupID);
+
+COMMIT;