MINOR: h2: send a real SETTINGS frame based on the configuration

MINOR: h2: send a real SETTINGS frame based on the configuration

An initial settings frame is emitted upon receipt of the connection
preface, which takes care of configured values. These settings are
only emitted when they differ from the protocol's default value :

  - header_table_size (defaults to 4096)
  - initial_window_size (defaults to 65535)
  - max_concurrent_streams (defaults to unlimited)
  - max_frame_size (defaults to 16384)

The max frame size is a copy of tune.bufsize. Clients will most often
reject values lower than 16384 and currently there's no trivial way to
check if H2 is going to be used at boot time.