New generic Ledger RPC handler.
Refactor away RPCHandler::doRpcCommand
RPCHandler::doCommand becomes free function
RPC::doCommand no longer returns a value.
Adjust old handler's signature to correspond to the new handler.
RPC::doCommand is now fully generic.
RPC handling now returns Status.
Make sure that handlers always return Json::objectValue.
Get rid of explicit template instantiation
Extract out fillHandler in RPCHandler.
Object::Root now owns its Writer.
New RPC::executeRPC function handles string output.
[fold] JsonWriter
[fold] ServerHandler
New RPC handling in ServerHandlerImpl.