srm: Fix authorization for srmv1 requests

Addresses an authorization regression in SRMv1 support.

When AuthorizationRecord was replaced by DcacheUser, SRMv1 authorization
broke. The SRMv1 code relies on SRMUser#equals to determine whether
the request query or update came from the same user that created the
request in the first place. Since DcacheUser doesn't implement equals,
this check always fails.

Rather than implement DcacheUser#equals, the SRM code is updated to
compare the IDs of the users instead. This SRM specific user identifier
is what is stored in the database, and in contrast to the DcacheUser
record, the ID can be restored from disk. The ID also ignores principals
like the Origin, which means that a request query or update will be
authorized even when issued from a different client host.

The patch only affects srmv1. Apparantly srmv2 doesn't do any
authorization checks.

Target: trunk
Request: 2.7
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6023/
(cherry picked from commit f7b11cd1a7d1a488dda1fb64fad15ca2c8009fa6)
Merge pull request #199 from gbehrmann/fix/2.7/rb6023

srm: Fix authorization for srmv1 requests