RPC: near-parity with JSON-RPC 2.0 specification

[redsh4de]

What

Brings the RPC closer to compliance with the JSON-RPC 2.0 specification

Not implemented are support for negative ids and batch requests (#694). The latter could be done after the rate limit changes in #685

Why

Correctness

Where

rpc

How

• Executes notifications without returning a JSON-RPC response
• Distinguishes missing id from explicit "id": null
• Returns -32700 Parse error for malformed JSON
• Returns -32600 Invalid Request for invalid request structures
• Returns -32601 Method not found for unknown methods
• Returns -32602 Invalid params for malformed method parameters
• Returns -32603 Internal error for internal failures
• Uses "id": null when the request ID cannot be determined