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>
(cherry picked from commit 354ee6d427e3ec8fc926d7fb793141a127733258)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>