* < clientKey payload [header]
* Route 'payload' to the client identified by 'clientKey'.
* The optional 'header' argument is a stringified JSON object,
* which will be written to the HTTP response header
* In case of multiple requests from a single client, the
* oldest request will be serviced on arrival of message
* < clientKey payload [header]
* Route 'payload' to the client identified by 'clientKey'.
* The optional 'header' argument is a stringified JSON object,
* which will be written to the HTTP response header
* In case of multiple requests from a single client, the
* oldest request will be serviced on arrival of message