This fixes a bug where a chunked response would be partially read and then hang forever waiting for another chunk, and adds additional debug logging to the request process.
- Use reqwest's async client
- Convert items_iter() to a futures::Stream
- make Mastodon client an Arc smart pointer, removing the need for OwnedPage.
- remove MastodonClient and HttpSender traits; these can be re-added once async trait fns are stabilized
- make EventStream a futures::Stream