Thinner, lighter yagnats wrapper.

- New NATSConn interface closely mirrors nats.Conn, allowing most of the
implementation to be removed and minimizing discrepancies with standard
nats client
- yagnats.Connect() and fakeyagnats.Connect() methods vend instances of
this interface, allowing for easy faking in tests
- Ping and AddReconnectedCB convenience methods

[#79120166]