Backwards compatibility for v1 peers

[kurtkilpela]

RSR protocol v2 is not backwards compatible with v1 clients. This means, a server and all connected clients must simultaneously migrate to protocol v2 at the same time.

Backwards compatibility for v1 clients should be assessed and added if it can be done.

[kurtkilpela]

Protocol V2 changes affect:

• RsrServiceSnapshot>>#encode:using:
• RsrServiceSnapshot>>#decode:using:
• RsrTemplateResolver
• RsrProtocolVersionNegotiation(Client|Server)

[kurtkilpela]

The move to protocol v2 is small. The breaking change is the switch to passing the template name and desired persona (client or server) to be created on the other side of the Connection. Previously, the name of the desired Client or Server class was put out onto the wire.

Right now, classes such as RsrServiceSnapshot handle encoding themselves using an encoder. I think this is wrong. This should be a double-dispatch and the encoder (or decoder) should handle the mapping between each object and its wire format. This would permit the use of a V1 and a V2 codec, each of which implements that specific version of the wire protocol.

[martinmcclure]

Reference also the "transports" design for encoding/decoding, which will be required in order to support encrypted communications.

[kurtkilpela]

Support merged in #171