Refactor RPC implementation

[vidya88]

@Tian-xi commented on May 8

1. UDP transport will be removed.
2. TCP transport will be merged into Node.

Hence, RPC implementation has to be refactored. It must contain only two public objects:

1. RPC - rpc server, based on net/rpc.
2. RPCClient - rpc client, based on net/rpc.

There is no need to split methods of RPCClient into 3 separate groups, like RootRPC, NodeRPC
and TransportRPC.

Note, that net/rpc is frozen.