Sync or Async dispatch (#83)

Sync or Async dispatch (#83)

* Support thread-pooled dispatch

* Add ConcurrencyBackend.run

* Initial work towards support byte-iterators on sync request data

* Test case for byte iterator content

* byte iterator support for RequestData

* Add BaseResponse

* Bridge sync/async data in SyncResponse

* Add BaseClient

* SyncResponse -> Response

* Tweaking type annotation

* Distinct classes for Request, AsyncRequest

* Tweak is_streaming, content in BaseRequest

* Stream handling moves to client

* Handle mediating to AsyncResponse from a standard sync Dispatcher class

* Working on thread-pooled dispatcher

* Support threaded dispatch, inc. streaming requests/responses

* Increase test coverage

* Coverage and tweaks

* Include Accept and User-Agent headers by default