GossipRouter: major overhaul

Things to be done:

• Use of Streamable rather than Externalizable
• Currently we use 1 thread per request rather than a thread pool, e.g. Executor !
• Better logging and diagnostics (JMX)
• Use of NIO to prevent the 1 thread/client scalability issue
• GossipClient and RouterStub should use common, shared code to communicate with the GossipRouter, currently there are exceptions because these 2 classes don't even use the same network format ! Besides, we can refactor redundant code into a common superclass
• Use of ConnectionTable(NIO) ?
• Add configuration options for GossipRouter's sockets, e.g. send/receive buffer size, nagling, keep_alive etc
• Support of UDP