ah owns rxbuf

This is intended to solve a longstanding problem with the
relationship between http/1.1 keep-alive and the service
loop.

Ah now contain an rx buffer which is used during header
processing, and the ah may not be detached from the wsi
until the rx buffer is exhausted.

Having the rx buffer in the ah means we can delay using the
rx until a later service loop.

Ah which have pending rx force POLLIN service on the wsi
they are attached to automatically, so we can interleave
general service / connections with draining each ah rx
buffer.

The possible http/1.1 situations and their dispositions are:

 1) exactly one set of http headers come.  After processing,
    the ah is detached since no pending rx left.  If more
    headers come later, a fresh ah is aqcuired when available
    and the rx flow control blocks the read until then.

 2) more that one whole set of headers come and we remain in
    http mode (no upgrade).  The ah is left attached and
    returns to the service loop after the first set of headers.
    We will get forced service due to the ah having pending
    content (respecting flowcontrol) and process the pending
    rx in the ah.  If we use it all up, we will detach the
    ah.

 3) one set of http headers come with ws traffic appended.
    We service the headers, do the upgrade, and keep the ah
    until the remaining ws content is used.  When we
    exhausted the ws traffix in the ah rx buffer, we
    detach the ah.

Since there can be any amount of http/1.1 pipelining on a
connection, and each may be expensive to service, it's now
enforced there is a return to the service loop after each
header set is serviced on a connection.

When I added the forced service for ah with pending buffering,
I added support for it to the windows plat code.  However this
is untested.

Signed-off-by: Andy Green <andy.green@linaro.org>