[master] Add handler extension callbacks after DNS resolution and SSL/TLS handshaking

Resolves #732 by adding two additional callbacks to the `AsyncHandlerExtensions` class: `onDnsResolved` and `onSslHandshakeCompleted`.

I made an effort to ensure that no additional tests are failing over what was already failing on master.

Implemented with @kstokoz.
Hye, can you add the license header? Thanks