Add $wgStatsDBMartEnabled variable

This changeset includes more fine-grained control over what part of statsdb is disabled and i s required in response to a P2 Ops are working right now.

/cc @macbre @kvas-damian 
Are you sure that Pokémon Exception Handling[1] is best solution here?
http://blog.codinghorror.com/new-programming-jargon/