For h1 / ws, a combination of removing POLLIN wait and
stashing any unused rx lets us immediately respond to
rx flow control requests in a simple and effective way,
because the tcp socket is the stream.
But for muxed protocols like h2, that technique cannot
be used because we cannot silence the whole bundle of
streams because one can't handle any more rx dynamically.
There are control frames and content for other streams
serialized inbetween the flow controlled stream content.
We have no choice but to read to so we can see the other
things. Therefore for muxed protocols like h2 and spdy,
rx flow control boils down to tx credit manipulation
on individual streams to staunch the flow at the peer.
However that requires a round trip to take effect, any
transmitted packets that were in flight before the tx credit
reduction arrives at the remote peer are still going to come
and have to be dealt with by adding them to the stash.
This patch introduces lws_buflist scatter-gather type
buffer management for rxflow handling, so we can append
buffer segments in a linked-list to handle whatever rx
is unavoidably in flight on a stream that is trying to
assert rx flow control.
Since new roles may be incompatible with http, add support for
alpn names at the role struct, automatic generation of the
default list of alpn names that servers advertise, and the
ability to override the used alpn names per-vhost and per-
client connection.
This not only lets you modulate visibility or use of h2,
but also enables vhosts that only offer non-http roles,
as well as restricting http role vhosts to only alpn
identifiers related to http roles.
