Fix regression which causes slow-downs on packet loss, close connection cleanly and extract the channels out

• Fix regression introduced in 2.5.0 where packet loss could lead to the
  connection to become very slow (#157, MagnusS, @talex5, @yomimono and
  @balrajsingh)
• Improve the tests: more logging, more tracing and compile to native code when
  available, etc (@MagnusS and @talex5)
• Do not raise Invalid_argument("Lwt.wakeup_result") everytime a connection
  is closed. Also now pass the raised exceptions to Lwt.async_exception_hook
  instead of ignoring them transparently, so the user can decide to shutdown
  its application if something wrong happens (#153, #156, @yomomino and @talex5)
• The channel library now lives in a separate repository and is released
  separately (#159, @samoht)