Add partial duplicate nullifier checks to transaction verifier

[arya2]

Motivation

The write block task in zebra-state is currently responsible for validating that there are no duplicate nullifiers in candidate blocks before they are committed to the state, but it checks this sequentially, one block at a time. It could be faster to check at least part of the chain in parallel in the transaction verifier so Zebra can often check only the non-finalized and in-memory state.

Note

There are currently many other opportunities to optimize Zebra's performance, their impact and expected implementation effort by comparison should be considered before this is scheduled.