Bug 13645: Cache the DBIx connection

Bug 13645: Cache the DBIx connection

We don't want to recreate a new connection to the DB every time we want
a new schema.

This patch creates a $database package level variable on the same way
it's done in C4::Context for $dbh.

Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Mason James <mtj@kohaaloha.com>