CLEANUP: backend: invert the condition to start the mux in connect_server()

CLEANUP: backend: invert the condition to start the mux in connect_server()

Instead of trying to switch from delayed start to instant start based
on a single condition, let's do the opposite and preset the condition
to instant start and detect what could cause it to be delayed, thus
falling back to the slow mode. The condition remains exactly the
inverted one and better matches the comment about ALPN being the only
cause of such a delay.