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.
7 jobs for rfc7675 in 2 minutes and 42 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #4548456
build

00:00:26

passed #4548458
1809 windows docker
build msvc amd64

00:02:37

passed #4548459
1809 windows docker
build msvc x86

00:02:37

failed #4548457
1809 windows docker allowed to fail
build msys2

00:00:55

 
  Test
passed #4548462
doc-and-install

00:01:09

passed #4548460
test

00:01:07

passed #4548461
test valgrind

00:01:07

 
Name Stage Failure
failed
build msys2 Build
Git branch:   13-2-stable
GO version: go1.13.8
Built: 2020-07-20T06:24:03+0000
OS/Arch: windows/amd64
Uploading artifacts...
WARNING: build/meson-logs/: no matching files
WARNING: build/build.ninja: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1