Replace rusk.persist() method

[herr-seppia]

Summary

With the land of piecrust, the rusk.persist() method is now a noop, since every accept/finalize is always persisted.
Unluckily, all the dusk-blockchain sync was incepted around that method.
(See also: dusk-network/dusk-blockchain#1530 and dusk-network/dusk-blockchain#1532)

Said that, current dusk-blockchain implementation is not usable with a piecrust version of rusk.

Possible solution design or implementation

To avoid completely wasting time on upgrading grpcAPI just to fit only dusk-blockchain need, we should at least upgrade those API with a logic that is reusable by the rusk full node.
So, I'm for changing the current state grpcAPI with the following

• Execute(from_root, txs[]) -> new_root, executed_txs[], invalid_txs[] //forget session
• Verify(from_root, txs[], expected_root) -> bool //forget session
• Persist(from_root, txs[], expected_root) -> () // Replace the previous Accept/Finalize calls. It executes txs and updates the current commit only if root is equal to the expected
• Finalize(from, to) --> () // Squash commits and update base_root to to if base_root==from
• Revert(to) -> () //remove all inner.commits between to and current_commit and replace current_commit with to

[goshawk-3]

Unluckily, all the dusk-blockchain sync was incepted around that method

I'm not certain this is true:

For the record, Persist grpc API was added as an optimization. (see also #572).

If we now are back to the strategy where we persist on each accept/finalize call, there should be no issue with dusk-blockchain sync if we always call StoreBlock(block, persisted = true).

[herr-seppia]

See also #970