Add tests for connection events

This is groundwork for adding more events in #732.

Sadly I couldn’t put these higher up in the test hierarchy, because the
Grizzly provider doesn’t seem to fire most events.
Add additional handler extension callbacks

Resolves #732.
Add license header to EventCollectingHandler
Merge pull request #747 from barnardb/additional-events

[master] Add handler extension callbacks after DNS resolution and SSL/TLS handshaking, close #732