jsonrpc: support positional params

[jchappelow]

Presently the params in the JSON-RPC request must be named (an object where the keys are the param names). This is JSON-RPC 2.0 convention. JSON-RPC v1 was initially positional (an array).

Maybe we should also work with positional so for example "params": [] works with request that otherwise do not require any arguments. Or so that "params": ["blah", "asdf"] works where we would otherwise require "params": {"arg1": "blah", "arg2": "asdf"}.

Named parameters are much much easier for server to work with, and also way more friendly if it is desired to omit some args but not others. Still, positional arguments are still part of the JSON-RPC spec.