BUG/MINOR: mux-h1: only reset the session's timing info on the frontend side

BUG/MINOR: mux-h1: only reset the session's timing info on the frontend side

When an H1 stream is detached, h1s_finish_detach() resets the session's
accept date, handshake and idle times so that the next request on the same
connection reports the right values. But this is also performed for backend
H1 streams, whose <sess> is the client's session. For an H1 frontend it does
not matter because the frontend stream is detached afterwards and resets
these values again. But for H2 and H3 frontends, the session is shared by
all streams of the client connection and the mux sets these values itself
when creating each stream. In this case the backend detach overwrites them,
so the next stream created on the client connection reports a wrong
accept/request date and idle time (%tr, %Ti, %Tt in logs): they are counted
from the end of the previous exchange with the server instead of from the
previous stream as the H2/H3 muxes intend.

For instance, with an H2 frontend and an H1 server, a client sending two
requests on the same connection, the first one taking 1 second, logs
"Ti=0 Tt=0" for the second one, instead of "Ti=1002 Tt=1002" with an H2
server or with this patch.

The timing info are now only reset for frontend H1 streams.

This patch may be backported to all supported versions.