add streaming event data mode (6.0 rewrite, part 2)

[eli-darkly]

This adds the incremental loading feature that was one of the main motivations for the whole rewrite. As before, the implementation is closely based on the Java version in okhttp-eventsource. Briefly:

• By default, everything works the same as before except that there is no longer any "store the message data as a reference to an internal byte buffer" behavior— it just stores the data as a string. The intention is that if an application is going to be reading large enough data that the string type would be inefficient, then they will use this new mode instead.
• If you set StreamEventData(true), then when you receive a MessageEvent it contains a special stream, accessed via the DataStream property. When you read from this stream, it is really reading from the EventParser, consuming the value of every data: field up till it hits a blank line, at which point that stream returns EOF.
• As soon as you request another event, the DataStream for the previous event becomes invalid, and EventParser skips over any content that hasn't yet been consumed from that one.

One wrinkle with this, which is a necessary departure from the SSE spec, is that EventParser has to return the MessageEvent before it has seen anything past the first data: line. So at that point, it doesn't yet know for sure that there really will be a full event before the end of the stream— and, if any other fields like event: or id: appear after data:, it can't put them into the MessageEvent. This is not an issue for using EventSource in the LaunchDarkly SDKs, because 1. until all of the data is consumed we won't actually be storing any of it, just parsing it; 2. the LD services always send event: before data:; and 3. with the new ExpectFields option, the SDK can specify that it definitely needs to see an event: field and so if for some reason the fields were sent out of order, EventSource will just go back to buffering the full event like before.