NIO based client

I have refactored the client program (now still named Client1), used in the tests. It is now based on non-blocking I/O and uses it features such as buffers, socket channels, selectors and charset encoders/decoders.
Client program has a single thread doing I/O and three groups of other threads:
1) Encoder threads. Since all messages are strings, they have to be converted to byte arrays before being sent to the sockets. Encoder threads take stings as input and produce byte buffers ready to inject into sockets (this injection is done by the I/O thread). Encoder threads are implemented in the form of a thread pool executor with specified number of threads.
2) Decoder threads. Once data (in form of byte buffers) have been read from the sockets by I/O threads, they need to be converted to strings to be able to process by the application. This work is done by decoder threads.
3) Callback threads. When an application is subscribing to messages (using a filter), it passes the callback to the client program. This callback gets called by callback threads (implemented as a thread pool). Callbacks are also called to propagate session exceptions within the client.