BUG/MEDIUM: mux-h1: make sure we always have a timeout on front connections

BUG/MEDIUM: mux-h1: make sure we always have a timeout on front connections

Mux-h1 currently heavily relies on the presence of an upper stream, even
when waiting for a new request after one is being finished, and it's that
upper stream that's in charge of request and keep-alive timeouts for now.
But since recent commit 493d9dc6ba ("MEDIUM: mux-h1: do not blindly wake
up the tasklet at end of request anymore") that assumption was broken as
the purpose of this change was to avoid initiating processing of a request
when there's no data in the buffer. The side effect is that there's no more
timeout to handle the front connection, resulting in dead front connections
stacking up as clients get kicked off the net.

This fix makes sure we always enable the timeout when there's no stream
attached to the connection. It doesn't do this for back connections since
they may purposely be left idle.

No backport is needed as this bug was introduced in 2.2-dev4.