Replace MWLogger with MWLoggerFactory

Replace MWLogger with MWLoggerFactory

Time wounds all heels. During the code review for the PSR-3 logging
introduction, several people asked me why we needed a wrapper
for Psr\Log\LoggerInterface if the point was to use the standard. At the
time I was convinced that it would be better to introduce the dependency
via a wrapper class so that we could use the wrapper to patch over any
deficiencies that we might find in the PSR-3 API. After going on to work
on a project to disentangle other MediaWiki components from internal
project dependencies I have suddenly and clearly seen the error of my
ways.

We still need a logger factory as PSR-3 does not specify
a standard mechanism for creating Psr\Log\LoggerInterface instances. My
solution is to convert MWLogger into MWLoggerFactory to retain a static
factory interface for creating PSR-3 loggers but remove the MWLogger
wrapper class itself in favor of direct exposure of
Psr\Log\LoggerInterface to the MediaWiki consumer classes.

Change-Id: Ie47467657dcf341991ada00827dca5e8eff95438