Sse migration from text to http package

This should address most of #205 and does address #209.

As mentioned in #209.  I moved the original classes and then created subclasses with the same name in the text.sse package.  These sub classes are marked as deprecated and will be removed, but they should provide backwards compatibility via casting to their super classes for anyone that happens to be using them in explicitly in their code. 
Is there value in keeping these classes since SSE can only be used in HTTP?

HTTP handlers use these only for referring to the handler instance.

If no, then we can just keep them deprecated & have a singleton `SSEInboundHandler` instance in the HTTP handler.

Same for client.