When pages are cached using some extra system (e.g. nc_staticfilecache)
it is useless to store these as well in whatever cache backend is used.
Thanks to a new hook, storage of a page in cache can be disabled
selectively.

Change-Id: Ie2e3fb61f06a4c0c3238d0644d45895b875686ff
Fixes: #39228
Releases: 6.0
Reviewed-on: http://review.typo3.org/13032
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Julien Henchoz
Tested-by: Julien Henchoz
Reviewed-by: Francois Suter
Tested-by: Francois Suter