Introduce a new client listener interface for the Java agent

Decouple the TCP client from the implementation of the LTTng Java agent.
Instead of using AbstractLttngAgent directly, the TCP client (and the
command subclasses) can deal with the new ILttngTcpClientListener
interface. The agent will implement this interface.

This will also allow easier testing of the TCP client and its protocol,
since test classess can now implement their own listener and verify the
contents of each command.

Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>