Add TPC worker pool

[matyasselmeci]

Upstream: xrootd#2525

[matyasselmeci]

Included in xrootd-5.8.4-1.4

(That build is not currently tagged into any repos, so we can test 5.8.4-1.4 and 5.8.4-1.3 separately.)

[matyasselmeci]

Upstream reports that they've been getting bug reports caused by this patch - see mailing list link. @bbockelm , suggestions? Should we pull this patch from the OSG set until it's fixed?