Populate the ZigZag parent's cache in advance for both forward and backward directions.

[schomatis]

rust-fil-proofs/storage-proofs/src/zigzag_graph.rs

Lines 359 to 365 in bb5a96e

	// TODO: Optimization: Evaluate providing an `all_parents` (and hence
	// `all_expanded_parents`) method that would return the entire cache
	// in a single lock operation, or at least (if the cache is not big enough)
	// it would allow to batch parents calculations with that single lock. Also,
	// since there is a reciprocity between forward and reversed parents,
	// we would only need to compute the parents in one direction and with
	// that fill both caches.

[schomatis]

(Bumping it to 3, this needs some more discussion about the design before it can be implemented.)

[dignifiedquire]

no more zigzag