Do not return before allowing the source to proceed to the next event.

When `enableRpc` is active, the `_processRpcTask()` method exits too
early, before `source.next()` can be called.  The problem is with
`return rpcTask.callback()`, when it should instead just invoke the
callback and proceed to call `source.next()` as usual.

By not calling `next()`, the message on the result queue is never
acknowledged, and no future responses will be received.