MINOR: connection: Preinstall the mux for non-ssl connect

MINOR: connection: Preinstall the mux for non-ssl connect

In the connect_server() function, there is an optim to install the mux as soon
as possible. It is possible if we can determine the mux to use from the
configuration only. For instance if the mux is explicitly specified or if no ALPN
is set. This patch adds a new condition to preinstall the mux for non-ssl
connection. In this case, by default, we always use the mux_pt for raw
connections and the mux-h1 for HTTP ones.

This patch is related to the issue #762. It may be backported to 2.2 (and
possibly as far as 1.9 if necessary).