chimera: Optimize schema migration

Also add empty rollback rules to allow the schema to be downgraded.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6494/
(cherry picked from commit e51e287e561544a635d4ebb4f1b07bb1ffc6ddd0)
[maven-release-plugin] prepare release 2.8.0-rc2+ndgf
[maven-release-plugin] prepare for next development iteration