BUG: http: re-enable TCP quick-ack upon incomplete HTTP requests

BUG: http: re-enable TCP quick-ack upon incomplete HTTP requests

By default we disable TCP quick-acking on HTTP requests so that we
avoid sending a pure ACK immediately followed by the HTTP response.
However, if the client sends an incomplete request in a short packet,
its TCP stack might wait for this packet to be ACKed before sending
the rest of the request, delaying incoming requests by up to 40-200ms.

We can detect this undesirable situation when parsing the request :
  - if an incomplete request is received
  - if a full request is received and uses chunked encoding or advertises
    a content-length larger than the data available in the buffer

In these situations, we re-enable TCP quick-ack if we had previously
disabled it.