RPC: add support for JSON-RPC 2.0-style request batching
RPC: add support for JSON-RPC 2.0-style request batching

If the top-level object is an array, it is assumed to be an array of
JSON-RPC requests.  An array is returned, containing one response (error or
not) per request, in the order submitted.

In a slight change in semantics, batched requests -always- return
an HTTP 200 OK status, even ones full of invalid or incorrect requests.

NeuCoin/neucoin,NeuCoin/neucoin,Magicking/neucoin,Magicking/neucoin,Magicking/neucoin,NeuCoin/neucoin,NeuCoin/neucoin,redfish64/nomiccoin,redfish64/nomiccoin,redfish64/nomiccoin,Magicking/neucoin,redfish64/nomiccoin,redfish64/nomiccoin,redfish64/nomiccoin