Function get_alt_block_extended_header_from_height 

pub fn get_alt_block_extended_header_from_height(
    db: &BlockchainDatabase,
    height: &AltBlockHeight,
    tx_ro: &Snapshot,
) -> DbResult<ExtendedBlockHeader>

Retrieves the [ExtendedBlockHeader] of the alt-block with an exact AltBlockHeight.

This function will look at only the blocks with the given AltBlockHeight::chain_id, no others even if they are technically part of this chain.