correctly parse dp4 responses

Between DP4 and DP3, response format changed:

• introduction of "requestID" and "clientContextID"
• resultset is now "results" (json array of result objects)
• there can be multiple "errors" and/or "warnings" (json arrays of objects) instead of results
• there's a "status" item after the results/errors/warnings
• there can be a "signature" block before the results (only if there's results)

Upgrade the QueryHandler to take those changes into account.