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.