Build efficient non-membership proofs for slow updates tree

[LHerskind]

Using the slow updates tree for a blacklist (such as USDC and USDT does) relies on the ability to prove non-membership.

The require the slow updates tree to be built such that it can efficiently perform both inclusion and non-membership checks.

We cannot directly use the indexed merkle tree, since this is also building on the basis of append-only elements. Which don't match our case where actors can be removed from blacklists.

Instead, we should explore the use of an altered indexed merkle tree, that support updates. The updates could practically be just a flag on whether or not the note is "active" or not. That way we can have append only elements such that the ordering is not messed up, and need to check the flag to see if active.

[nventuro]

Closed via #5954 as we got rid of slow updates.