fix(connection): openUri return promise-like on existing connection

[DiegoRBaquero]

Summary
Fixes #9335

[clhuang]

Suggested change

	this.then = (resolve) => {
	this.then = (resolve) => {
	delete this.then;
	delete this.catch;
	resolve(this);
	return this
	}

if you don't do this, if you try to call Promise.resolve(conn) or await conn your interpreter will go in an infinite loop of calling resolve recursively as it tries to flatten the promise, see #8810

[DiegoRBaquero]

makes, sense, fixed! However in this way, it's not possible to do conn.openUri(...).then().catch()

[clhuang]

I guess if you really wanted to you could do

this.then = (resolve) => {
  process.nextTick(() => {
    delete this.then;
    delete this.catch;
    resolve(this);
  });
  return this;
}

[vkarpov15]

Thanks for this PR, but we're going to make an alternative fix that's more in line with where we want to go with #8810. Since mongoose.connect() always returns a promise rather than something that can be used as a connection, we can change mongoose.connect() instead of openUri().