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/