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

Implements #732 for 1.9.x.
Merge pull request #746 from barnardb/1.9.x-addtional-events

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