Bug 17807: Use XSLT_Handler in Koha::OAI

Bug 17807: Use XSLT_Handler in Koha::OAI

Replacing some code in the OAI modules by a call to an existing module.
Note that the xmldoc format is used in the transform call to get a xml
document object.

The stylesheet method of Repository now only returns the name of the xsl
file to be used instead of a cached xslt object. Similar functionality
inside XSLT_Handler is used when calling transform.

Note: We still lack unit tests in this area. I did not see the need for
adding something for stylesheet since it only returns a simple string.
The other change is made in Record::new; there are no tests for this
module yet and the heart of the change here is actually tested already
in XSLT_Handler.t.

Note: I benchmarked calls to Repository in the old and the new situation
and did not see significant changes.

Test plan:
[1] Run t/db_dependent/OAI/Server.t
[2] Run oai.pl with ListRecords and marcxml.
[3] Run oai.pl with ListRecords and oai_dc.

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>