pool: Fix access time update

The bug causes access time update to happen on pool to pool and not on regular
downloads - the reverse is supposed to happen.

This affects sweeper garbage collection order, space cost calculation, and
access time update in the name space.

Target: trunk
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8234
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6564/