Add an FCV check so that an attempt to invoke $out/$merge in a mixed-version replica set will always return a useful error message when the read preference is non-primary. If the operation targets a binary version 4.4 node in FCV "4.2", then this new FCV check should trip. If the operation targets a binary version 4.2 node, then the operation will fail with a 'NotMaster' category error, just as it would in 4.2 and all previous versions.