poolmanager: Restore compatibility with pre 2.2.13 and earlier releases

dCache 2.2.14 introduced an incompatibility with previous versions. This
patch addresses the issue by restoring compatibility with version 2.2.13
and earlier.

Note that sites that already upgraded to dCache 2.2.14 will have to update
all doors and pool manager together: dCache 2.2.14 doors and pool manager
cannot be mixed with 2.2.13 or earlier nor 2.2.15 and later.

Target: 2.2
Require-notes: yes
Require-book: no
Merge pull request #145 from gbehrmann/fix/2.2/rb5873

poolmanager: Restore compatibility with pre 2.2.13 and earlier releases