Move the balancer status from the serverStatus section to a balancerStatus command

After some discussion it was decided that a more appropriate means to expose the balancer state is through the controlBalancer command instead of doing it through the serverStatus section on the config server.

Going the controlBalancer way we avoid the need for clients to have to talk directly to the config server or for the mongos serverStatus command to have to do network calls.

The status will be exposed through a new parameter called status, which will return the same fields, which are now in the serverStatus.balancer section.