Remove retention of bytes for a `Block`

[lehins]

It looks like we never need to deal with original bytes for a Block. It is only the Block header and Block body (TxSeq) that need to retain the bytes.
Consensus uses block header hash, not the block hash, because block header also encapsulates the hash of the block body. Hence, from the security point of view, block header hash is the block hash and as such we never need to compute the hash of the whole block.

This ticket tracks removal of bytes for the Block and an attempt to integrate into consensus in order to confirm that such removal is indeed safe.

[lehins]

Done in #4926