bulk insert support of asyncmy

[whitemay]

I noticed that asyncmy now support executemany to improve performance.
I think it is time to use executemany now. bulk inerting sooooooooo slow.

[aminalaee]

The execute_many right now is running execute in a loop: https://github.com/encode/databases/blob/master/databases/backends/postgres.py#L226

Feel free to create a PR for that driver or any other that supports it.