srm: Fix transition ordering for restored requests

The transition time isn't precise enough to serve as a key or sorting.
Instead we use the element id which we know is allocated sequentially.

Target: trunk
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6972/