Optimize execution of user event handlers

[andyrichardson]

About

For now, all user-provided event handlers are executed sequentially.

await onSubscribe(); // User side-effect - blocking
// ...
// subscriptionless handling logic

It would be nice to have the option between:

• sequential execution (user handler -> subscriptionless handler -> response)
• concurrent execution (user handler and subscriptionless handler -> response)

Note: onConnectionInit handler should always be executed sequentially for authentication purposes