Check availability cache when serving block by root requests #5093
Description
Description
We aren't quire compliant with this change from the previous pre-release (v1.4.0-beta.5) ethereum/consensus-specs#3551
We are serving blobs in accordance to the spec because we're accessing the data availability cache. With blocks, we're using the early attestation cache, which only holds one block at a time, the one we currently think will be head. Using the availability cache instead should get us fully compliant with this spec change.