pallet-mmr: fix offchain db for sync from zero (#12498)

* pallet-mmr: cosmetic improvements

* pallet-mmr: fix offchain storage for initial sync

* address review comments

* pallet-mmr: change offchain fork-resistant key to `(prefix, pos, parent_hash)`

Do this so that both canon and fork-resitant keys have the same
`(prefix, pos).encode()` prefix. Might be useful in the future if we'd
be able to to "get" offchain db entries using key prefixes as well.

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

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