Provide a backwards compatibility shim for MWLogger

Provide a backwards compatibility shim for MWLogger

MWLogger was renamed MWLoggerFactory and reduced to a static utility
class in Ie474676. Reintroduce an MWLogger that proxies the public
static methods of MWLoggerFactory to ease the transition of users who
have implemented $wgMWLoggerDefaultSpi configurations that reference
MWLogger.

As noted in the class, this is a temporary bandaid that should be ripped
off before 1.25 becomes an official release.

Change-Id: Iaccb78a510c60aab2ff20a9aa7c0869699657388