Update pow_chain to dict and reuse it instead of calling get_pow_block

[hwwhww]

Issue

Per #2636 (comment) discussion:
If the validator already maintains all PoW blocks, they don't really have to call get_pow_block(block.parent_hash) (which we implied it's a JSON-RPC call) to get the parent block.

How did I fix it

1. As @mkalinin suggested, we can change the abstract pow_chain structure to Dict[Hash32, PowBlock] so that we can query the block by hash easily.
2. Change parent = get_pow_block(block.parent_hash) to parent = pow_chain[block.parent_hash].
   • Note: KeyError is considered invalid.

It turns out get_terminal_pow_block is much more concise with this change! ✨

[djrtwo]

lgtm

[mkalinin]

Looks good to me! 👍