Bug 16636: [QA Follow-up] Make BakerTaylor plack safe

Bug 16636: [QA Follow-up] Make BakerTaylor plack safe

Initialize file level lexicals each call. Do not call _initialize
outside the module.
Adjust test by mocking preferences.

Test plan:
Run t/db_dependent/External_BakerTaylor.t.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Tested module with trivial script under Plack/memcached by toggling
the associated preferences.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 5361440f6538127c8c3ec61d963d5f0692c64a71)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
(cherry picked from commit c8ef873ee9fe97210d00d450a19f6ad52036b400)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>