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/
(cherry picked from commit b5f77a6707762b21d8e9227fdd846933f5ee6878)
Merge pull request #178 from gbehrmann/fix/2.7/rb5944

srm: Make Scheduler and ModifiableQueue type aware