ROADMAP: remove HTTP/2 multiplexing - its here now

   To decide: if we need to bundle parts of the nghttp2 stuff that probably

   won't be shipped by many distros.

- multiplexing/pipelining

   Add a bit to `CURLMOPT_PIPELINING` which then makes libcurl attempt to

   multiplex HTTP/2 connections to the same host, using the HTTP1 pipelining

   host matching logic. Doing multiple transfers to the same host using the

   multi interface will then make them multiplexed. By default, this will

   handle flow-control for the streams in an automatic fashion.

   With multiplexed streams, we also need to make sure that we terminate

   streams correctly when a single easy handle is removed from such a

   connection.

   Since HTTP/2 is designed to always use a single connection for each host

   (origin really), it seems fair that with "pipelining" enabled there will be

   no extra checks for number of streams etc added to the connection apart

   from the maximum number set over the HTTP/2 protocol (the remote sets a

   maximum) and only if that is reached, another connection gets created.

- stream properties API

   Provide options for setting priorities and dependencies among the streams