agent: implement support for RFC7675 - Consent Freshness
Specified in https://tools.ietf.org/html/rfc7675
RFC 7675 is a slight modification of the existing keepalive connection checks that could be enabled manually or were used with the GOOGLE compatibility mode.
Slight differences from the existing keepalive connection checks include:
- an additional consent expiry timer instead of relying on all binding requests to succeed.
- 403: 'Forbidden' stun error-code which revokes consent with immediate effect.