Listening to events on the DB object is deprecated in v3.x and will b…

…e unsupported in v4 of the mongodb drivers. Adjusted remaining listeneers to use the DB client object, as per the MongoDB Driver API docs

lib/connection.js

@@ -929,7 +929,7 @@ function _setClient(conn, client, options, dbName) {
         }
       });
 
-      db.on('close', function() {
+      client.on('close', function() {
         const type = get(db, 's.topology.s.description.type', '');
         if (type !== 'ReplicaSetWithPrimary') {
           // Implicitly emits 'disconnected'
@@ -945,7 +945,7 @@ function _setClient(conn, client, options, dbName) {
   });
 
   if (!options.useUnifiedTopology) {
-    db.on('reconnect', function() {
+    client.on('reconnect', function() {
       _handleReconnect();
     });
 
@@ -965,7 +965,7 @@ function _setClient(conn, client, options, dbName) {
     });
   }
   if (!options.useUnifiedTopology) {
-    db.on('close', function() {
+    client.on('close', function() {
       // Implicitly emits 'disconnected'
       conn.readyState = STATES.disconnected;
     });
@@ -984,7 +984,7 @@ function _setClient(conn, client, options, dbName) {
       }
     });
 
-    db.on('timeout', function() {
+    client.on('timeout', function() {
       conn.emit('timeout');
     });
   }