BUG/MINOR: mux-h2: refuse to allocate a stream with too high an ID

BUG/MINOR: mux-h2: refuse to allocate a stream with too high an ID

One of the reasons for the excessive number of aborted requests when a
server sets a limit on the highest stream ID is that we don't check
this limit while allocating a new stream.

This patch does this at two locations :
  - when a backend stream is allocated, we verify that there are still
    IDs left ;
  - when the ID is assigned, we verify that it's not higher than the
    advertised limit.

This should be backported to 1.9.