Fix streaming Reader race condition

[douglaswth]

• There was a race condition between creating a streaming.Reader with NewReader and calling its Subscribe where events could be consumed before there is any channel to send them to.
• To fix this, the default behavior is now to only start the reader's read goroutine when Subscribe is called after it has inserted a channel that will receive events.
• Alternatively, there is now an option WithReaderStartExplicitly which will only start the read goroutine when the new Start is called allowing multiple calls to Subscribe before receiving events.