Avoid use of deprecated wfGetLBFactory() function

Was deprecated in 1.27 and this extension requires MW 1.31 which
MediaWikiServices::getInstance()->getDBLoadBalancerFactory() is
available (in 1.28), so this change is fine.

No MediaWiki extension uses this function anymore and this is the
last extension making use of this function.

Usage:

https://codesearch.wmflabs.org/search/?q=%5B%5E%3E%5DwfGetLBFactory%5C(&i=nope&files=&repos=

Bug: T42485
Change-Id: Id415d70aa0090c01ea04c3156aecd76caa8e83f8