Rewrite async bits using "await"

[simu]

I just realized that javascript (at least webextensions, cf. https://github.com/mdn/webextensions-examples/blob/3fb6017d4387d86f29bc702afc2d630d1a121262/session-state/background.js#L20) has a "await" keyword to wait for an async operation to complete.

This might make some of the storage code easier to understand.