Tree states to support per-slot state diffs

[jimmygchen]

Description

Currently the HDiff logic does not support per-slot state diffs, and it would be nice to support this.

This requires the following (from @michaelsproul):

• Re-writing the epoch-centric code in HDiff to work with slots instead.
• Changing the on-disk DB format as the above change will break it.
• Add some config for the "hierarchy exponents" to the OnDiskStoreConfig

Relevant code:

lighthouse/beacon_node/store/src/hdiff.rs

Lines 206 to 255 in 079cd67

	impl HierarchyConfig {
	pub fn to_moduli(&self) -> Result<HierarchyModuli, Error> {
	self.validate()?;
	let moduli = self.exponents.iter().map(|n| 1 << n).collect();
	Ok(HierarchyModuli { moduli })
	}
	pub fn validate(&self) -> Result<(), Error> {
	if self.exponents.len() > 2
	&& self
	.exponents
	.iter()
	.tuple_windows()
	.all(|(small, big)| small < big && *big < u64::BITS as u8)
	{
	Ok(())
	} else {
	Err(Error::InvalidHierarchy)
	}
	}
	}
	impl HierarchyModuli {
	pub fn storage_strategy(&self, epoch: Epoch) -> Result<StorageStrategy, Error> {
	let last = self.moduli.last().copied().ok_or(Error::InvalidHierarchy)?;
	if epoch % last == 0 {
	return Ok(StorageStrategy::Snapshot);
	}
	let diff_from = self.moduli.iter().rev().find_map(|&n| {
	(epoch % n == 0).then(|| {
	// Diff from the previous state.
	(epoch - 1) / n * n
	})
	});
	Ok(diff_from.map_or(StorageStrategy::Nothing, StorageStrategy::DiffFrom))
	}
	/// Return the smallest epoch greater than or equal to `epoch` at which a full snapshot should
	/// be stored.
	pub fn next_snapshot_epoch(&self, epoch: Epoch) -> Result<Epoch, Error> {
	let last = self.moduli.last().copied().ok_or(Error::InvalidHierarchy)?;
	if epoch % last == 0 {
	Ok(epoch)
	} else {
	Ok((epoch / last + 1) * last)
	}
	}
	}

@michaelsproul I'll likely need some help understanding this, so adding you if you have some time to pair or chat!

[jimmygchen]

Completed in #4652 🥳