FO: Check server validity before setting status
The list of resolved servers is allocated on the back end context and
kept in the fo_service structure. However, a single request often
resolves a server and keeps a pointer until the end of a request and
only then gives feedback about the server based on the request result.
This presents a big race condition in case the SRV resolution is used.
When there are requests coming in in parallel, it is possible that an
incoming request will invalidate a server until another request that
holds a pointer to the original server is able to give a feedback.
This patch simply checks if a server is in the list of servers
maintained by a service before reading its status.