MINOR: mux-h2: simplify the wake up code in h2_rcv_buf()

MINOR: mux-h2: simplify the wake up code in h2_rcv_buf()

The code used to decide when to restart reading is far from being trivial
and will cause trouble after the forthcoming changes: it checks if the
current stream is the same that is being demuxed, and only if so, wakes
the demux to restart reading. Once streams will start to use multiple
buffers, this condition will make no sense anymore. Actually the real
reason is split into two steps:
  - detect if the demux is currently blocked on the current stream, and
    if so remove SFULL
  - detect if any demux blocking flags were removed during the operations,
    and if so, wake demuxing.

For now this doesn't change anything.