Optimize Database Size with Shorter Hashes

[adamkrellenstein]

Combine/refactor/drop/truncate ledger_hash, messages_hash, txlist_hash and event_hash (which should be event_id). We don't need nearly so much entropy there—we need collision resistance only.

We can do more things like avoid storing the tx_hash in tables of transactions. It's a foreign key, so it doesn't use much extra space on its own, but it bloats the messages table. Obv. we have to be careful not to sacrifice performance with this step. EDIT: tx_hash isn't even a foreign key here! I think it should be, along with tx_index, block_index, etc.