It would have been more convenient to store the quota size in the
ArtifactCache, except that the scheduler needs the quota size and
doesn't have access to the ArtifactCache.

Jonathan/cache cache size

See merge request !679

Jonathan/cascache cell var from loop

See merge request !686

Valentin David authored Aug 08, 2018 and Qinusty committed Aug 21, 2018

Issue was introduced by 171e803f (include directive) and the fix was
found courtesy of @Qinusty. This fixes also the include
feature. Because elements are to be serialized, the included fragments
need to use copy_tree when loaded.

Related to #470.