http: modify timeout behaviour

In the previous version, the timeout for the connection could be
set only after async_connect while the request is sending.
This means if async_connect is slow (it can happen as if there is
an ipv6 endpoint, it can try to resolve it during dozens of seconds
before falling back on ipv4).

In this patch, it moves the timeout callback in the request. This
callback will be set just before async_connect, to be sure that we
are using the callback while connecting.