Parallelise note commitment tree point conversions

[teor2345]

Motivation

We can speed up note commitment tree updates by running point conversions in parallel with tree updates.

Designs

Run the following operations in parallel rayon iterators or threads:

• each point conversion, or batches of point conversions, in any order, in multiple threads (cm_x.into())
• appending notes, in blockchain order, in a single thread (self.inner.append(...))

We can start appending notes as soon as the first point conversion is completed.

This change can be made for Sprout, Sapling, and Orchard. Sprout is a lower priority because it's not currently being used much.

Orchard example:

zebra/zebra-chain/src/orchard/tree.rs

Lines 309 to 310 in 267db7f

	pub fn append(&mut self, cm_x: NoteCommitmentUpdate) -> Result<(), NoteCommitmentTreeError> {
	if self.inner.append(&cm_x.into()) {

zcashd example of out of order note insertion using shardtree:
zcash/incrementalmerkletree#50

[teor2345]

This is currently optional. We've talked about it a few times, so I wanted to write it down so we had a record.

But I think we can close it, unless we decide we need it to fix #5420, and that #5420 is an important enough fix to schedule soon.

Edit: or we decide we want to fix #5709, which we need to fix before this ticket will help with sync speed.