[GEOS-6445] IOUtils.xStreamPersist fails if /tmp on a different filesystem
GEOS-6542 CAS doesn't work with GeoServer
Merge pull request #566 from bencaradocdavies/geos-6445

[GEOS-6445] IOUtils.xStreamPersist fails if /tmp on different filesystem
Cumulative fix for

GEOS-6109 CAS login request exposes internal server URL
GEOS-6110 CAS logout request is incorrect
Migrate use of GeoserverDataDirectory to GeoServerResourceLoader.

Fix Factory pre-startup reference to GeoServerResourceLoader

Previous implementation always got null ... and never noticed.
Remove references to GeoserverDataDirectory singleton
Remove GeoserverDataDirectory singleton

Revise based on feedback

GeoServerExtensions init methods added to ease test case writing.