Bug 23387: Add caching to C4::ClassSource

Bug 23387: Add caching to C4::ClassSource

These routines ultimately need to be moved to the Koha namespace, for now though,
we can reduce look ups during import and batch modification by caching the values
here for each request

To test:
1 - prove -v t/db_dependent/ClassSources.t
2 - Import some records with items, confirm cn_sort values correctly built
3 - Edit some items, confirm cn_Sort correclty built

Signed-off-by: Phil Ringnalda <phil@chetcolibrary.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>