Add get_treestate method to ChainIndex

[idky137]

Currently there is no way to fetch the treestate as of a given block height with the ChainIndex, this is required by zallet to continue their wallet dev.

Current plan:

• Add get_treestate to BlockChainSource.
• Cherry pick get_block_height code from Zainod chain index #496 to simplify block hash -> block height conversions.
• Add get_treestate to ChainIndex
• Update / Add tests.