Remove SetCallBackKey in favor of authenticator appdata

The SetCallBackKey RPC is more complicated than necessary to
achieve its goal of binding a key for secure callbacks to the
RPCs generating those callbacks.

Instead of having an extra identity field in the token (that is
to be cached in the server's per-connection private data) and a
separate lookup table for mapping the callback identity (and cache
manager UUID) to a callback key, just provide the callback key (and
token) directly in the authenticator alongside the token.  The
authenticator is constructed anew for each connection, so a new key
can be used for each connection if desired.

Accordingly, remove the cb_id field from the token format, and
the remaining references to it.

Change-Id: I4a65756cd73174c2960086393186232fa53af487
Remove the start_time field from the token format

It essentially serves no purpose.  At best, it would help ensure
time consistency between different servers, but only in some cases,
and would be more likely to result in user frustration than corrective
action.

The start_time field in the rxgk response structure (in the core
rxgk document) is needed, as it is an input to the transport key
derivation process.  However, because that start_time is unique
per-connection and a single token can be used to create multiple
connections, there can be no correspondence between the start_time
in the response and the start_time in the token.

Change-Id: I7cdabba22c58d2d5064e93b35e03aea446d4df7f
Roll changelog

Change-Id: I04aa6952170d345092f1b415ee38bace0f4f8b9e
Make draft-wilkinson-afs3-rxgk-afs-06

Change-Id: Ic4065e3fd6194837140f1f4dbab3f5fd00ab5aaa
Token lifetime/bytelife are unsigned

We made them unsigned in the negotiation structures; they should
be unsigned here as well.

Change-Id: Iff73ec87a1f5918279c25390cdd3887328fda632