BUG/MEDIUM: resolvers: Use the right server to init DNS streams

During TCP nameservers parsing, the wrong server was selected to initialize
the DNS stream. After the server addition, the first server of the proxy was
selected instead of the last one. So when several TCP nameservers was
defined, only the first one was properly initialized. All others was using
the server created for the first one.

The regression was introduced when the server insertion in the proxy's list
was inverted. The change was not made on the resolvers part. The
proxy_last_server() function must be used instead.

It is a 3.5-specific issue. No backported needed.