README.http2: refreshed and added multiplexing info

HTTP2 with libcurl

 Spec: http://tools.ietf.org/html/draft-ietf-httpbis-http2

 Spec: http://www.rfc-editor.org/rfc/rfc7540.txt

 Document explaining it: http://daniel.haxx.se/http2/

  than HTTP/1.1 (which we implement on our own) and that nghttp2 is an already

  existing and well functional library.

  Right now, nghttp2 implements http2 draft-14

  We require at least version 0.6.0

  We require at least version 1.0.0.

 Over an http:// URL

    GnuTLS:   ALPN

    PolarSSL: ALPN

HTTP Alternative Services

Multiplexing

  Alt-Svc is a suggested extension with a corresponding frame (ALTSVC) in

  http2 that tells the client about an alternative "route" to the same content

  for the same origin server that you get the response from. A browser or

  long-living client can use that hint to create a new connection

  asynchronously.  For libcurl, we may introduce a way to bring such clues to

  the applicaton and/or let a subsequent request use the alternate route

  automatically. Spec:

  http://tools.ietf.org/html/draft-ietf-httpbis-alt-svc-05

  Starting in 7.43.0, libcurl fully supports HTTP/2 multiplexing, which is the

  term for doing multiple independent transfers over the same physical TCP

  connection.

  To take advantage of multiplexing, you need to use the multi interface and

  set CURLMOPT_PIPELINING to CURLPIPE_MULTIPLEX. With that bit set, libcurl

  will attempt to re-use existing HTTP/2 connections and just add a new stream

  over that when doing subsequent parallel requests.

  While libcurl sets up a connection to a HTTP server there is a period during

  which it doesn't know if it can pipeline or do multiplexing and if you add

  new transfers in that period, libcurl will default to start new connections

  for those transfers. With the new option CURLOPT_PIPEWAIT (added in 7.43.0),

  you can ask that a transfer should rather wait and see in case there's a

  connection for the same host in progress that might end up being possible to

  multiplex on. It favours keeping the number of connections low to the cost

  of slightly longer time to first byte transfered.

Applications

  curl offers the --http2 command line option to enable use of http2

TODO:

HTTP Alternative Services

  - Implement multiplexing

  Alt-Svc is a suggested extension with a corresponding frame (ALTSVC) in

  http2 that tells the client about an alternative "route" to the same content

  for the same origin server that you get the response from. A browser or

  long-living client can use that hint to create a new connection

  asynchronously.  For libcurl, we may introduce a way to bring such clues to

  the applicaton and/or let a subsequent request use the alternate route

  automatically. Spec:

  http://tools.ietf.org/html/draft-ietf-httpbis-alt-svc-05

  - How to tell libcurl when using the multi interface that all or some of the

    handles are allowed to re-use the same physical connection. Can we just

    re-use existing pipelining logic?

TODO:

  - Provide API to set priorities / dependencies of individual streams