pool: Fix memory leak in xrootd and httpd movers

Addresses a slow memory leak that occurs whenever more than one httpd
or more than one xrootd transfer is started on a pool. Also reduces
http/xrootd server component startup overhead.

Target: trunk
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5695/
(cherry picked from commit 7679d8ddd80872b9301869846f71f413976281c5)
Merge pull request #119 from gbehrmann/fix/2.6/rb5695

pool: Fix memory leak in xrootd and http movers