Enable write-through mode for cache

[matyasselmeci]

See xrootd#2472. This is scheduled to be in XRootD 5.9.0 but we'd like to use it for Pelican sooner.

[matyasselmeci]

Included in xrootd-5.8.4-1.2, which is now in osg-release.

[matyasselmeci]

Looks like it did not end up in upstream's 5.9.0 and the upstream milestone has been removed.