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It would be tedious to update the JSON message specifications to add tagged fields to each structure.  Similarly, we don't wan want to manually annotate each string, buffer, or array that should now be serialized in a more efficient way.  Instead, we should simply have the concept of "flexible versions." Any version of a message that is a "flexible version" has the changes described above.

In order to have flexible version support across all requests and responses, we will bump the version of all requests and responses.  The new versions will be flexible.  (This version bump may be implemented earlier for some message types than others, depending on implementation considerations.)

RequestHeader Version 1

Requests within a "flexible version" will have a new version of the request header.  The new RequestHeader version will be version 1, superseding version 0.  In this new version, the RequestHeader's ClientId string will be a COMPACT_STRING rather than  STRING.  Additionally, the header will contain space for tagged fields at the end.  Supporting tagged fields in the request header will give us a natural place to put additional information that is common to all requests.

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