Rework the way messages are exchanged between server and client (msgpack + chunking)

[alesgenova]

• Use msgpack for serialization of the messages between server and client
  • slightly reduced message size
  • binary values can directly be in the object serialized/deserialized (no need to use previous attachment mechanism)
• Chunk messages
  • avoids the websocket connection to close unexpectedly when a large message is exchanged
  • use existing environmental variable WSLINK_MAX_MSG_SIZE to determine the chunk size