BUG/MINOR: sink: detect and warn when using "send-proxy" options with ring servers

BUG/MINOR: sink: detect and warn when using "send-proxy" options with ring servers

using "send-proxy" or "send-proxy-v2" option on a ring server is not
relevant nor supported. Worse, on 2.4 it causes haproxy process to
crash as reported in GH #2965.

Let's be more explicit about the fact that this keyword is not supported
under "ring" context by ignoring the option and emitting a warning message
to inform the user about that.

Ideally, we should do the same for peers and log servers. The proper way
would be to check servers options during postparsing but we currently lack
proper cross-type server postparsing hooks. This will come later and thus
will give us a chance to perform the compatibilty checks for server
options depending on proxy type. But for now let's simply fix the "ring"
case since it is the only one that's known to cause a crash.

It may be backported to all stable versions.