fix(change-streams): ensure a majority read concern on initial agg

Author: mbroadst

lib/change_stream.js

@@ -149,6 +149,7 @@ var buildChangeStreamAggregationCommand = function (serverConfig, namespace, pip
   var command = {
     aggregate : namespace.collection,
     pipeline : changeStreamPipeline,
+    readConcern: { level: 'majority' },
     cursor: {
       batchSize: options.batchSize || 1
     }