RpcResult improvements

[eigen-value]

For const-correctness and thread safety of RpcResult objects

• .result code should be better locked by an object level mutex / an atomic cas that guarantees the single execution
• RpcResult template should hold the returned RType result so that multiple calls return the same result by ref It can't be done because .result is a template by design (eg what happens if it is called multiple times but with different types? .result(int) then .result(str) the effect would be undefined)
• Document better

Minor, but preferred:

• rename RpcResult as RpcCall
• the object param error should be better initialized so that if it is accessed before the call is executed it is a self explanatory one like GENERIC_ERR with "RPC call not executed".
• Check const-correctness of both RpcResult and Bridge