Execute should return an awaitable with AsyncioExecutor

The current graphql.execution.execute.execute implementation forces AsyncioExecutor to use loop.run_until_complete in wait_until_finished. This is a bit ugly.

It would be better to let the executor decide what is returned from the execute method. It can be given a function to call after the data is ready as an argument.

With this AsyncioExecutor could return an awaitable and execute would be used as one would expect

`result = await execute(schema, ast, executor=AsyncioExecutor())`

It could even have an argument to get the new way to keep compatibility with existing uses.

This would also make TwistedExecutor much easier to implement.

graphql.execution.execute.execute から始めて、AsyncioExecutor が wait_until_finished をどのように使用しているかを調べ、次に TwistedExecutor に関連する executor の統合ポイントを比較します。executor が post-data callback をどのように提供するか、また互換性をどのように維持するかを定義します。AsyncioExecutor が await execute で使用可能な awaitable を返せるようになり、既存の使用方法も引き続きサポートされれば完了です。

索引モデルが issue の本文から書いたものです。