RPC registration and resolving relevant handlers

[naleeha]

Things to solve

• When Java application tries to add their own RPCHandler and implement RpcHandler.scala it asks to implement existing methods. This is as this class is not a pure interface but Java does not support Traits.
• When resolving relevant request handling logic for View port specific RPC message, it match based on method names and number of parameter. This means if you were to rename the method name in RpcHandler class, message sent for UI has to change too. Would be better to have a clear contract between UI and Server that is not tied to internals of the server
• When resolving relevant request handling logic for global RPC message, it match based on interface name and method name. Would be better to have a clear contract between UI and Server that is not tied to internals of the server

[naleeha]

Potential option

One option is to explicitly register the RPC handler with the KEY. RPC Request message will contain the KEY
See usages of DefaultRpcHandler.scala for example of this. e.g. PersonRpcHandler.java (#1465)

Future follow ups raised as new issue #1502