Improved, fixed, tested

Improved

• New method NiolBuffer.subWrite(maxLength)
• New method NiolBuffer.lsub(begin, end)
• Precise definition of what the sub methods return.
• Better handling of large messages
• Upgrade to scala 2.12.4
• More documentation

Fixed

• Fixed TcpServer.start()
• Fixed CircularBuffer.skipRead(n)
• Fixed CircularBuffer.subRead(maxLength)
• Fixed CompositeBuffer.subRead(maxLength)
• In CompositeBuffer, readAvail and writeAvail are updated when necessary
• NioBasedBuffer now redefines the sub method to a faster and more adapted code
• In ClientAttach, the consistency of the SelectionKey's interestOps is now guaranteed by a synchronized block.