Public key pinning
When libcurl4-openssl-dev >= 7.39.0, add support to the C client via the
options struct for public key pinning.
http://curl.haxx.se/libcurl/c/CURLOPT_PINNEDPUBLICKEY.html

Client ID rotation
Add an API endpoint to invalidate an old client_id and switch to a new one
with the same profile.
Add support in both the JS and C clients to periodically issue this and
rotate client_id, then to rotate instance_id (which already just works).