Bug 16431: Use Koha::Cache to cache marc subfield structure

Bug 16431: Use Koha::Cache to cache marc subfield structure

The marc subfield structure is currently cached using a global variable
of C4::Context. The infos are retrieved every time a new context is
created.
This patch suggests to use Koha::Cache instead.

To achieve this goal, a new subroutine is created
C4::Biblio::GetMarcSubfieldStructure, it will be called from code which
needs to get the marc subfield structure. GetMarcFromKohaField,
GetMarcSubfieldStructureFromKohaField, TransformKohaToMarc and
_get_inverted_marc_field_map are modified accordingly and the cache is cleared
when the table is updated (from the 3 pl scripts modified by this patch).

The caching done in C4::Context (marcfromkohafield) is removed.

Test plan:
Play with the marc subfield structure (in the administration module),
then add and edit records and make sure everything went fine.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Everything works as expected on my functional tests. I'm really happy to see the
patch introduces relevant tests for previously untested functions.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>