MINOR: mux_quic: do not perform unnecessary timeout handling on BE side

MINOR: mux_quic: do not perform unnecessary timeout handling on BE side

MUX implements a timeout for HTTP keep-alive which monitors the delay
between two HTTP requests. This is only applicable for frontend
connections, as on the backend side idle connections can be kept in the
server pool. In QUIC mux, this timeout relies on QCC <idle_start> which
is refresh when the last request is terminated.

This patch modifies the refresh operation so that it is only performed
for frontend connections. This is not strictly necessary but the timeout
timeout management is now clearer and it eliminates an unnecessary
operation for backend connections.

Similarly, http-request timeout is also only applicable for frontend
connections. This relies on qcs_wait_http_req() function. A request QCS
is inserted in <opening_list> until the headers are received. This is
unnecessary on the backend side so this is excluded as well.