Move features of MatrixHttpApi related to application-services into subclass

まだ誰も着手していません。

See discussion on #143.

In general, I want to move the sdk more towards a set of composable classes with a clear api for extending them. Something where parameters specific to application-service usage live either in an additional kwarg `extras` or in slurped kwargs `**extras` seems like the right direction for this.

Issues to be resolved still:

- [ ] What if somebody wants to combine functionality available on two different subclasses of `MatrixHttpApi` (e.g. application-service support and async as in #168)? We should support composing those together somehow.

Maybe in addition to swapping out `_send` we should have a list of decorators that get applied to `_send`?

cc @Cadair

まず MatrixHttpApi と #143 の議論を読み、その後 #168 の関連する async 提案を確認します。application-service の機能がコンポーザブルなサブクラスまたは拡張に移され、issue で未解決となっている application-service と async の動作の組み合わせに明確な解決策が定義されれば完了です。

索引モデルが issue の本文から書いたものです。