If two create_block_template are called at nearly the same time,
and a block is added at nearly the same time, this could happen:
- the blockchain top block is B0
- thread 1 enters create_block_template, takes blockchain lock
- thread 1 creates a fresh block referencing prev block B0
- thread 1 releases blockchain lock
- thread 0 adds a new block
- thread 0 enters create_block_template
- thread 0 updates block template
- thread 1 takes txpool lock and continues creating block template
- thread 1 overwrites block template with previous data