Semantic versioning

[osteele]

Design a process for handling the breaking protocol changes in #135, #140. If it's flag day, write up a plan so that we can execute it. If it's one of the others, document it (and add supporting code, if any), so that we can re-use it or start form that for the next round of breaking changes.

This could be:

• Add a version flag to the request. Listed for completeness.
• Add a /v2 prefix. This is a widely-used pattern.
• Two-phase deprecation. Add the new flags to the server; deploy; modify the client; deploy; remove the old flags from the server; deploy at leisure.
• Flag day — everyone must upgrade. This requires inter-team coordination, coordinating multi-app deployments, and some downtime.