Formalize the Ping Protocol

[sappenin]

This PR has been dormant for a very long time, but I'm reintroducing it for a few reasons:

1. We have a new proposals process, and formalizing "ping" still seems like a good proposal.
2. This has been on my TODO list forever
3. Uni-directional mode seems useful in "send-only" modes (see discussion here)
4. Bi-directional mode is deployed in the JS connector and also seems useful in certain production networks.

Links for Context

• Interledger forum discussion (here).
• Previous PR from long ago (here).

[sappenin]

I am unsure what mechanism the receiver would know that this is specifically a PING packet

In unidirectional mode, the receiver knows this is a ping packet based upon two things: 1) The packet is destined to the Connector's operating address; 2) The packet uses the well-known condition.

In my mind, there's no data payload needed in unidirectional flow. In bi-directional flow, the data payload is needed to signal receiver where to send the pong packets to.

[matdehaast]

I am unsure what mechanism the receiver would know that this is specifically a PING packet

In unidirectional mode, the receiver knows this is a ping packet based upon two things: 1) The packet is destined to the Connector's operating address; 2) The packet uses the well-known condition.

In my mind, there's no data payload needed in unidirectional flow. In bi-directional flow, the data payload is needed to signal receiver where to send the pong packets to.

Thanks that makes sense.

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