objectcache: Use Psr\Log\LoggerInterface instead of wfDebug

objectcache: Use Psr\Log\LoggerInterface instead of wfDebug

ObjectCache::newFromParams() will handle a 'loggroup' parameter
specially, getting an instance of \Psr\Log\LoggerInterface to
pass to the BagsOStuff.

BagOStuff implementations can use $this->logger which will be
an implementation of \Psr\Log\LoggerInterface. As this is set in
BagOStuff::__construct(), all subclasses must now call the
parent constructor.

The goal of this is to make the logging in BagOStuff non-MediaWiki
specific, in the hopes of separating it out into a separate library
in the future.

Change-Id: I8a8e278e6f028814499d8457d6d5341d03eabc7a