SERVER-20214 we are removing support for majority read concern level in these operations because we cannot write in that mode. This ticket is to reenable support.
The current plan is to emulate majority read concern level by waiting for w:majority in the portions of those operations that write (aggregations including $out and all map reduces) and keep the read concern level for the portions that don't (aggregations not including $out including all work from the shard phase). It is unclear right now which category inline map reduce falls under. Part of this work is to ensure that this approach is valid.