nomination-pools: optimize re-bonding unclaimed rewards.

[kianenigma]

There is an option in pools to directly bond your unclaimed rewards. This is currently coded, as of #11669, such that is basically like a transfer. The funds are literally transferred to your account, and taken out again.

We can and should optimize this such that the account of the pool member is not touched in this process. Although, since they pay the fee anyways, their account data is already in overlay. Some code cleanup can also be done via doing this.

substrate/frame/nomination-pools/src/lib.rs

Lines 1496 to 1502 in 82287b0

	reward_pool.update_records(bonded_pool.id, bonded_pool.points);
	// TODO: optimize this to not touch the free balance of `who ` at all in benchmarks.
	// Currently, bonding rewards is like a batch. In the same PR, also make this function
	// take a boolean argument that make it either 100% pure (no storage update), or make it
	// also emit event and do the transfer.
	let claimed =
	Self::do_reward_payout(&who, &mut member, &mut bonded_pool, &mut reward_pool)?;

[Szegoo]

I am going to give this a try.

[kianenigma]

We tried this with @Szegoo in the linked PR and it proved to complicate the code quite a lot. Not something that we want to do for now. If we want to re-visit, this is the best we've got so far: #11767 (comment)