For obvious compatibility reasons, the over-the-wire protocol needs to be
versioned.  Since we're using HTTP, it makes sense to simply version the
URLs used.

The scheme is pretty simple:

    http://host:port/operation/version(/extra)?

where "version" is a simple integer.

Open questions:

  - Are there operations where we don't want to require a version?

  - Is there a use for a "300 Multiple Choices" HTTP return code?

  - If we always try the client-optimal version first and then back off
    linearly, we'll want to cache the version that succeeded so we don't
    spend all of our time trying versions that don't exist.

  - Alternatively, we can call "versions" to find out what versions the
    server supports for each operation, and cache that.