refactor: deprecate Db.on and Db.once

Deprecates methods inherited from EventEmitter, Db.on and Db.once,
which have been removed in 4.0.

NODE-1726

Author: emadum

lib/db.js

@@ -211,6 +211,14 @@ function Db(databaseName, topology, options) {
 
 inherits(Db, EventEmitter);
 
+Db.prototype.on = deprecate(function() {
+  return Db.super_.prototype.on.apply(this, arguments);
+}, 'Listening to events on the Db class has been deprecated and will be removed in the next major version.');
+
+Db.prototype.once = deprecate(function() {
+  return Db.super_.prototype.once.apply(this, arguments);
+}, 'Listening to events on the Db class has been deprecated and will be removed in the next major version.');
+
 // Topology
 Object.defineProperty(Db.prototype, 'topology', {
   enumerable: true,