Added new default configuration option for mongo db: useUnifiedTopology (#162)

Author: odin16

lib/databases/mongodb.js

@@ -96,6 +96,7 @@ function Mongo(options) {
   if (isNew) {
     defaultOpt.autoReconnect = false;
     defaultOpt.useNewUrlParser = true;
+    defaultOpt.useUnifiedTopology = true;
     _.defaults(options.options, defaultOpt);
   } else {
     defaultOpt.auto_reconnect = false;