pool: Move post processing to post execution service

PoolIORequest currently both holds transfer properties and
properties independent of any particular transfer.

This patch seperates the post processing related functionality
out of PoolIORequest and into the post execution service.

Target: trunk
Require-book: no
Require-notes: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5429/