agent: implement support for RFC7675 - Consent Freshness

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.