Blocks: Use async functions in the classic checkout

[RadoslavGeorgiev]

In https://github.com/Automattic/woocommerce-payments/blob/master/client/checkout/classic/index.js there are various instances where async functions should be used, now that the file is compiled/transpiled with webpack and Babel.

For some reason I could not get babel to transform async calls to Promises for backwards compatibility. We should figure out how to fix it and do it.

Example: https://github.com/Automattic/woocommerce-payments/blob/master/client/checkout/classic/index.js#L136