Remove unnecessary witnesses

[sug0]

Updating the witness map doesn't remove incremental witnesses of leaf positions in the commitment tree related with notes that have already been spent (i.e. whose nullifiers have been revealed).

namada-masp-indexer/chain/src/services/masp.rs

Lines 10 to 48 in 6cc34cd

	/// Update the merkle tree of witnesses the first time we
	/// scan a new MASP transaction.
	pub fn update_witness_map(
	commitment_tree: &CommitmentTree,
	tx_notes_index: &mut TxNoteMap,
	witness_map: &WitnessMap,
	indexed_tx: IndexedTx,
	shielded: &namada_core::masp_primitives::transaction::Transaction,
	) -> anyhow::Result<()> {
	let mut note_pos = commitment_tree.size();
	tx_notes_index.insert(indexed_tx, note_pos);
	for so in shielded
	.sapling_bundle()
	.map_or(&vec![], |x| &x.shielded_outputs)
	{
	// Create merkle tree leaf node from note commitment
	let node = Node::new(so.cmu.to_repr());
	// Update each merkle tree in the witness map with the latest
	// addition
	witness_map.update(node).map_err(|note_pos| {
	anyhow::anyhow!("Witness map is full at note position {note_pos}")
	})?;
	if !commitment_tree.append(node) {
	anyhow::bail!("Note commitment tree is full");
	}
	// Finally, make it easier to construct merkle paths to this new
	// note
	let witness =
	IncrementalWitness::<Node>::from_tree(&commitment_tree.get_tree());
	witness_map.insert(note_pos, witness);
	note_pos += 1;
	}
	Ok(())
	}

If we could map nullifiers to note positions, we could stop updating their witnesses, thereby speeding up update_witness_map, and also decreasing the load on the database (since we won't need to store as much data, and likewise clients won't need to fetch as much data).