Remove previous_block_hash from Block Stream BlockHeader

[jasperpotts]

Problem

Having previous_block_hash in the Block Stream BlockHeader causes a timing problem that prevents optimal parallel thread pipelining in the consensus node. To get the block hash we have to have finished hashing the state of the previous block. This can take as long as it takes to execute all transactions in the next block. Because of that we do not want to hold up creating the block header until that hash is ready.

Solution

We already also have the hash of the previous block in BlockProof.previous_block_root_hash so we can just remove BlockHeader.previous_block_hash. This should then remove the synchronization requirement from services and allow more optimal parallel thread pipelining. The detailed comment for explaining the hash should be moved to BlockProof.previous_block_root_hash so we don't loose it.

[jasperpotts]

@steven-sheehy @xin-hedera You might need to update mirror node based on this change.