client/mmr: persisting gadget state across runs

Fixes #12780

* client/mmr: on init do canonicalization catch-up

* client/mmr: add more tests

* client/mmr: persist gadget progress in aux db

* client/mmr: add more tests

* client/mmr: replace async_std with tokio

* remove leftover comment

* address review comments

Signed-off-by: acatangiu <adrian@parity.io>