Move features of MatrixHttpApi related to application-services into subclass

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

首先阅读 MatrixHttpApi 和 #143 中的讨论，然后查看 #168 中相关的 async 提案。当 application-service 功能被移入可组合的子类或扩展中，并且 issue 中尚未解决的 application-service 与 async 行为组合问题有了明确的解决方案时，即视为完成。