add create_time and access_time to asset db for mysql, as well

as the code to update these at the appropriate time.  This isn't
surfaced in AssetBase yet.

Change the replace into to an insert into for asset create.  Assets
are not supposed to be updatable, and the replace into is more
expensive.

From: Sean Dague <sdague@gmail.com>

OpenSim/Data/MySQL/Resources/005_AssetStore.sql

@@ -0,0 +1,6 @@
+BEGIN;
+
+ALTER TABLE assets add create_time integer default 0;
+ALTER TABLE assets add access_time integer default 0;
+
+COMMIT;