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/