Add send channels

[Henauxg]

Add send channels to Quinnet:

• Changes:

  • OrderedReliable
  • UnorderedReliable
  • Unreliable
  • Add 2 new important API calls on server & client: open_channeland close_channel (and a few minor utilities such as some try_ functions)
  • Add new error types and rename ambiguous ChannelClosed to InternalChannelClosed

• Task list:

  • [x] Channels on the client
  • [x] Channels on the server
  • [x] Default channels creation
  • [x] Keep the current simple API working by default
  • [x] Open & close channels at runtime
  • [x] Graceful shutdown, wait for all channels to be properly flushed and finished
  • [x] Share channels implementation
  • [x] Documentation

• Implement channel types:

  • [x] OrderedReliable
  • [x] UnorderedReliable
  • [x] Unreliable