Consistency Tests for Connection Management and Events

[neonphog]

Tx5 encapsulates some complexity in terms of connection management, and downstream crates depend on consistent behavior. We may need to build in some affordances for triggering certain conditions in order to even test some scenarios.

We need to build consistency tests for the following:

• Fallback SBD Messaging Management, Events, and Testing #130
• Preflight callbacks are invoked always and only when a new connection is established. Specifically, they are only invoked once whether we are able to upgrade to webrtc or if we are switching to fallback messaging given the consistent behavior of the bullet point above. If a "Disconnected" event is emitted (indicating the connection is closed), then preflight checks are again run next time a connection is established with that peer.
• A "Connected" event is always and only emitted once after preflight callbacks are successful given the consistent behavior of the bullet point above.