BUG/MEDIUM: haterm: Properly handle end of request and end of response

BUG/MEDIUM: haterm: Properly handle end of request and end of response

There were several issues with the handling of end of the request or end of
response. The main problem was about the request draining.

To help to fix these issues, two flags were introduced:

 * HS_ST_HTTP_EOM_RCVD: to know the request was fully received
 * HS_ST_HTTP_EOM_SENT: to know the response was fully sent

Thanks to these flags some parts were reviewed and simplified.

In the I/O callback function, outside of any error, the hstream task is now
woken when one of the direction is not finished or when there are still some
data in a buffer.

The function hstream_must_drain() was reworked to properly drain request
with no content-length before replying.

The condition to wake hstream up to drain the request after replying was
also reworked, and moved ouside of the else block. Indeed, it must also be
evaluated when the response was fully sent in one call, when request headers
were processed.

Finally, the condition to shut the hstream was slighly adapted to use the
new flags. In addition, we now rely on se_shutdown().