Adding async-await for generateId function

[efkan]

When providing an Id from an in-memory database, accessing the database over the network and getting the id may cause to skip id assignment.

In this case id will be undefined. Because of this reason engine.io fall into an endless loop.
To overcome this issue async-await combination can be used.

The kind of change this PR does introduce

• a new feature
• a bug fix
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

generateId may be skipped as undefined because of id generating operations.

New behaviour

By using await option, handshake method can be waited to complete generateId() function.

Other information (e.g. related issues)

However, async-await combination doesn't work on former Node.js versions.

[darrachequesne]

How about:

this.generateId(req, function (id) {
  // ...
});

[efkan]

@darrachequesne , it didn't come to mind!

On the other hand I would have to wrap the rest of the code into a callback function.
I guess async-await provides more readability. But actually it doesn't matter for me.

Thank you!

[efkan]

I couldn't pass the tests on Travis.
So, I'm closing the PR.

[efkan]

@darrachequesne , I've tried to commit my PR. Unfortunately with a callback it throws different error. They may be reletad with this keyword. Meybe not.

As a result, I gave up for now.