Like |sendAsyncMessage()|, except blocks the sender until all
listeners of the message have been invoked. Returns an array
containing return values from each listener invoked.
Like |sendSyncMessage()|, except re-entrant. New RPC messages may be
issued even if, earlier on the call stack, we are waiting for a reply
to an earlier sendRpcMessage() call.
Both sendSyncMessage and sendRpcMessage will block until a reply is
received, but they may be temporarily interrupted to process an urgent
incoming message (such as a CPOW request).