Change UserMessage to class and remove defaultUserMessage class.
Add new helper classes.
Refactor package structure. Add helper classes for simple encapsulation
of forest core.
Refactor package structure of core.message.
Remove unnecessary classes for keepalive logic.
Correct keepalive implementation.
Add simple wrapper classes of NioSocketConnector in helper package, and
make necessary refactor.
Refactor package structure and add factory class for NioProcessor. Also
add build file for auto-packaging.
Remove some System.out for debugging.
Add KeepAliveIoSession interface and remove IoSessionLocal interface.
And make necessary changes to related sources.
Add file transfer function to forest and apply necessary changes.
Complete the implementation of ForestFileSender and ForestFileReceiver,
and make necessary related changes.