dcache-xrootd: Extend and improve xrootd access log plugin

Adds more information to the access log plugin output, and refines the existing
output.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: https://rb.dcache.org/r/7470/
Request: 2.11
Request: 2.10