[MEDIUM] http: make the analyser not rely on msg being initialized anymore

[MEDIUM] http: make the analyser not rely on msg being initialized anymore

The HTTP parser needed the msg structure to hold pre-initialized pointers.
This causes a trouble with keep-alive because if some data is still in the
buffer, the pointers can be anywhere after the data and later become invalid
when the buffer gets realigned.

It was not needed to rely on that since we have two valid information
in the buffer itself :

  - buf->lr : last visited place
  - buf->w + buf->send_max : beginning of next message

So by doing the maths only on those values, we can avoid doing tricks
on msg->som.