MEDIUM: check: include server address and port in the send-state header

MEDIUM: check: include server address and port in the send-state header

This fixes an issue that occurs when backend servers run on different
addresses or ports and you wish to healthcheck them via a consistent
port. For example, if you allocate backends dynamically as containers
that expose different ports and you use an inetd based healthchecking
component that runs on a dedicated port.

By adding the server address and port to the send-state header, the
healthcheck component can deduce which address and port to check by
reading the X-Haproxy-Server-State header out of the healthcheck and
parsing out the address and port.