TcpConnection: forward received data to handler immediately without concatenating buffers

For these reasons:

 - pipeline effect will allow to start processing on the first part of the
   data immediately in parallel
 - data in `Received` messages is now always a simple ByteStrings which will
   improve iteration speed in the next layer
 - code becomes simpler
Merge pull request #1294 from spray/tcp-connection-immediate-Received-message

TcpConnection: forward received data to handler immediately without concatenating buffers