srm: Make Scheduler and ModifiableQueue type aware

Addresses the problem that ModifiableQueue restores jobs without
knowing their type, thus incurring extra overhead as several
job storages have to be consulted.

Target: trunk
Request: 2.7
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5944/