Consider protocol "minor" versioning strategy

[analogrelay]

For client-to-server pings, we need a way to express to the server that the client supports some optional feature so the server knows it can adjust behavior. This is just one example of possible future needs here. For now, the server only marks the client as eligible for being timed out when it has sent a single ping, and we're going to make sure the client always sends a single ping. However, I want to revisit this in 2.2 and consider a possible "minor versioning" strategy for protocols that can be expressed during the handshake.

The 2.1 RTM server was designed to ignore extra properties in the HandshakeRequest payload which means we can use that to add an additional minorVersion field to the payload which is used to detect minor protocol changes. We should consider adding something like that and then consider changing the server behavior around pings to use that.

I'm filing this because I don't want to block the client-to-server ping feature on this stuff right now.

[analogrelay]

This is going to be needed to support client-to-server streaming.