CachedStorage doesn't cache sequences, only digests

The CachedStorage has a cache by digest. However we sometimes need to access it without knowing the exact digest of the block.

The current implementation simply naively searches in the storage if the expected digest isn't known.

We should additionally cache by sequence and return the blocks by sequences.

Start by locating CachedStorage and tracing its digest-based lookup and the fallback search used when the digest is unknown. Add sequence-based caching so blocks can be returned by sequence without searching storage, then verify both digest and sequence lookups behave correctly.

Written by the indexing model from the issue text.