refactor chain to be map of blocks, traverse top to bottom

[thepiwo]

• replace list in chain worker with map
• reference blocks by hash in this map
• keep head_block_hash list additionally to map in genserver state (should be a list because there could be multiple valid head blocks in the future)
• to get list of blocks, traverse from latest, via prev_block_hash to the bottom
• refactor block validation to have only block and chainstate as parameter. get the prev_block by hash from the chain, and the traverse the chain to get blocks for difficulty check