Append up to 2000 block hashes for which to get headers.

def add_block_hash( self, block_hash ):
        """
        Append up to 2000 block hashes for which to get headers.
        """
        if len(self.block_hashes) > 2000:
            raise Exception("A getheaders request cannot have over 2000 block hashes")

        hash_num = int("0x" + block_hash, 16)
        
        bh = BlockHash()
        bh.block_hash = hash_num

        self.block_hashes.append( bh )
        self.hash_stop = hash_num