Does this crate need async IO after all?

[daladim]

The tokio tutorial says that "performing only a few web requests" is not a good reason to use tokio and async IO, and suggests to use the blocking version of reqwest instead.

Since kitchen_fridge only performs a few web requests, and in a very limited code region (almost only in Provider::sync()), maybe we could indeed drop all the async code.
Provider::sync() could indeed be blocking (but run in a separate thread if the users wants it not to block).

This would simplify the async traits exposed in the public API, and would ease the testing as well (that currently requires being built with a specific cargo feature)