poolmanager, doors: Don't retry stage requests on the same pool

Addresses a regression introduced between 1.9.12 and 2.2. The regression
means that poolmanager would not remember the previously tried stage pool
and thus could select the same pool twice in a row when retrying failed
stage requests.

The patches addresses this problem. To be effective, both pool manager
and doors have to be updated, although the patch does not break any
version compatibility.

Target: trunk
Request: 2.6
Request: 2.2
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/5678/
(cherry picked from commit 4174110f6d6083908a8937e42e1c4969596e53ac)
Merge pull request #114 from gbehrmann/fix/2.6/rb5678

poolmanager, doors: Don't retry stage requests on the same pool