MINOR: mux-quic: correctly implement backend timeout

MINOR: mux-quic: correctly implement backend timeout

qcc_refresh_timeout() is the function called on QUIC MUX activity. Its
purpose is to update the timeout by selecting the correct value
depending on the connection state.

Prior to this patch, backend connections were mostly ignored by the
function. However, the default server timeout was selecting as a
fallback. This is incompatible with backend connections reuse.

This patch fixes timeout applied on backend connections. Only values
specific to frontend which are http-request and http-keep-alive timeouts
are now ignored for a backend connection. Also, fallback timeout is only
used for frontend connections.

This patch ensures that an idle backend connection won't be deleted due
to server timeout. This is necessary for proper connection reuse which
will be implemented in a future patch.