pnfsmanager: Fix compatibility with 2.6 pools

2.6 pools send PnfsGetStorageInfoMessage upon issuing the 'rh restore' command.
This message has been removed and thus pool compatibility with 2.6 is broken.
This patch adds a minimal version of that message back to ensure compatibility
with old pools.

Target: trunk
Request: 2.8
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6696/
(cherry picked from commit 3158ca2f03b6bda16c1723d52873882b86721454)
Merge pull request #458 from gbehrmann/fix/2.8/rb6696

pnfsmanager: Fix compatibility with 2.6 pools