doors: Shorten session and initiator strings

Our session strings have been redundant in the sense that we added
uniquness counters at several levels. This has lead to fairly long
session strings.

Doors have also added their domain name to the session string, even
when doors usually are well-known and the cell name is unique within
the dCache instance.

This patch cleans this up by avoiding adding redundant uniqueness
counters and the domain name when not needed.

This affects the session string that appears in log files and the
initiator strings that appear in billing.

Target: trunk
Require-notes: yes
Require-book: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7416/