Moving block successors to their own table.

[clemahieu]

When a new block is inserted, the previous block needs to be rewritten in order to update the sideband information which contains the block successor. This causes extra disk activity to copy and re-write the database page that contains the block.

This is a requirement in order to get the benefits from splitting the block table https://github.com/nanocurrency/nano-node/issues/4053.

This moves block successors out of block sideband and in to their own database table.