pool: Move scheduling state into SimpleIoScheduler

Scheduling state is kept in PoolIORequest although it is exclusively
maintained and used by the scheduler.

This patch moves scheduling related fields (scheduling state, scheduling
related time stamps) to PrioritizedRequest, an inner class of
SimpleIoScheduler. Thus scheduling information is now represented in
and maintained by the scheduler.

Target: trunk
Require-book: no
Require-notes: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/5424/
Merge branch 'master' of https://github.com/dCache/dcache