srm: Fix lock congestion

To support SRM_FILE_BUSY, both get and ls requests scan for active
put requests. This causes lock congestion as reading the SURL of
the put request acquires a read lock, while abort and put done calls
are processed with a write lock (and these calls trigger callouts
to pnfsmanager).

The read lock to access the surl is however unneeded as surl is a
final field. This patch removes the read-lock for surl access. The
patch also removes the read-lock on several other final fields, as
well as marking several fields that are not modified as final.

Target: trunk
Request: 2.9
Request: 2.8
Request: 2.7
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7054/
(cherry picked from commit 3ac48ab7b5e81965f900b9546e53f6d0e929a7b0)

Conflicts:
	modules/srm-server/src/main/java/org/dcache/srm/request/CopyFileRequest.java
	modules/srm-server/src/main/java/org/dcache/srm/request/PutFileRequest.java
Merge pull request #627 from gbehrmann/fix/2.8/rb7054

Fix/2.8/rb7054