Mempool should discard any transaction with repeated nullifier

[goshawk-3]

Describe what you want implemented
On transaction retrieval, Mempool should check if any of transaction nullifiers is already used by another transaction in mempool state. The check should be executed after a successful rusk.Preverify call.

Describe "Why" this is needed
This change will allow a block generator for passing transactions to EST without repeated nullifiers. Additionally, mempool will not need anymore to run a cumbersome discardUsedNullifiers at any block acceptance.