storage: check alt blocks when handling missing tx request in DB

[redsh4de]

What

Support alt blocks in TxsInBlock

Closes #580

Why

txs_in_block only searches the main chain at the moment. A peer can request missing transactions for a block that is on an alt chain - either from a reorg or because Cuprate broadcasts alt blocks as of #577

Where

storage/blockchain

How

• Rename existing get_alt_block to get_alt_block_information so it's more honest about it's return type (AltBlockInformation)
• New get_alt_block mirrors the main-chain equivalent (returns Block, is used in txs_in_block)
• Extend txs_in_block to try the main chain first, then fall back to alt-chain tables on KeyNotFound
• add and use a alt_block_height helper to DRY

[redsh4de]

this was made before the tapes db upgrade, aligned it just now

• renamed the prior get_alt_block to get_alt_block_information so it's more honest about its return type (returns AltBlockInformation)
• get_alt_block now is basically a mirror of get_block: also returns Block, and is used when looking up the missing txes