Write cursors to TextWriter directly instead of creating strings

- Write cursor manually to the TextWriter instead of using the JsonTextWriter.
- Changed GetCursor on ISubscriber to WriteCursor is Action<TextWriter>.
- Get rid of MakeCursor, MakeCursorFast and MakeCursorSlow since they're unneeded.
- Moved LongPollingTransport message bus logic into the transport itself.
- Removed Receive overload on ITransportConnection that was made specifically for LongPolling.
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