BUG/MEDIUM: http-client: Don't wake http-client applet if nothing was xferred

BUG/MEDIUM: http-client: Don't wake http-client applet if nothing was xferred

When data are transferred to or from the htt-pclient, the applet is
systematically woken up, even when no data are transferred. This could lead
to needlessly wakeups. When called from a lua script, if data are blocked
for a while, this leads to a wakeup ping-pong loop where the http-client
applet is woken up by the lua script which wakes back the script.

To fix the issue, in httpclient_req_xfer() and httpclient_res_xfer()
functions, we now take care to not wake the http-client applet up when no
data are transferred.

This patch must be backported as far as 2.6.