BUG/MINOR: mux-h1: Handle keep-alive timeout for idle frontend connections

BUG/MINOR: mux-h1: Handle keep-alive timeout for idle frontend connections

IDLE frontend connections have no stream attached. The stream is only
created when new data are received, when the parsing of the next request
starts. Thus the keep-alive timeout, handled into the HTTP analysers, is not
considered while nothing is received. But this is especially when this
timeout must be considered. Concretely the http-keep-alive is ignored while
no data are received. Only the client timeout is used. It will only be
considered on incomplete requests, if the http-request timeout is not set.

To fix the bug, the http-keep-alive timeout must be handled at the mux
level, for IDLE frontend connection only.

This patch should fix the issue #984. It must be backported as far as
2.2. On prior versions, the stream is created earlier. So, it is not a
problem, except if this behavior changes of course (it was an optim of the
2.2, but don't remember the commit).