Bug 16365: Selectively introduce GetMarcStructure() "unsafe" variant for better perfo...

Bug 16365: Selectively introduce GetMarcStructure() "unsafe" variant for better performance

GetMarcStructure() currently uses Koha::Cache in the "safe" mode
(returning deep copy of the result data structure by default), which
causes numerous performance issues in many Koha scripts. Switching
it to the "unsafe" mode globally (2nd patch from Bug 16140) resolves
those issues, but ensuring that it is regression-free (and that it
will stay that way in the future) is far from easy. This patch
proposes a bit more manageable solution, it introduces
a possibility to use "unsafe" variant selectively (only in those
places in the code where GetMarcStructure() is called repetitively).
That way, amount of the code that needs to be audited for possible
problems gets vastly reduced, without any performance trade-offs.

Test plan:
1) Have a look at the code and audit the parts affected by this patch
for possible regressions

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Amended the POD of GetMarcStructure, removing a TODO.

NOTE: GetAuthorisedValueDesc, as called in C4::XSLT::transformMARCXML4XSLT
and by GetISBDView, GetMarcAuthors in C4::Biblio, may autovivify some hash
entries in tagslib.
Same for Koha/Filter/MARC/ViewPolicy.pm, sub filter.

No reason however to worry; our use of this structure in Koha does not
depend on the existence of intermediate hash keys. (We seem to be safe as
long as $tagslib->{$tag}->{$subfield}->{tab} and/or hidden are not filled.)

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>