New accumulators and MR operators should be FCV 4.4 only

$accumulator should be available only when FCV is kFullyUpgradedTo44.  We need to extend the REGISTER_ACCUMULATOR macro, similar to REGISTER_EXPRESSION_WITH_MIN_VERSION.

We can also mark $_internalJsReduce and $_internalJsEmit as 4.4 only.  These internal operators are part of the new mapReduce implementation, and mongos should only produce them on FCV 4.4.