[1.9.x] Add handler extension callbacks after DNS resolution and SSL/TLS handshaking

Implements #732 for 1.9.x by adding two additional callbacks to the `AsyncHandlerExtensions` class: `onDnsResolved` and `onSslHandshakeCompleted`.

There didn't seem to be any tests for the handler extensions callbacks, so I added a few in the first commit. They are limited to the Netty provider, because the Grizzly provider doesn't seem to support the handler extensions. The second commit updates the tests and implements the new callbacks.

Implemented with @kstokoz.