assets+tarodb: adopt versioning for asset updates+transfers

Nobody has claimed this yet.

Today whenever we go to modify an asset on disk (due to a transfer, import, etc) we just overwrite the asset fields in place, but hold onto the same asset primary key. In the future, it would be nice to be able to do things like query for the balances of a given asset at a given point in time (block hash).

In order to allow such queries we should:
* Add a version field to each asset
* Bump that each time we modify the asset
* Never delete the old managed UTXOs or chain_txns on disk (just update the primary key pointer)
* Always order by the version field descending (distinct and fetch last) for queries

With the above changes, we can find the balance at a given hash with:
* find the chain txn that has that hash
* join w/ managed utxo to find the one that matches
* join w/ asset to find the version that pointed to that utxo
* do the normal balance stuff

Start by tracing the asset, managed UTXO, and chain transaction persistence and query entry points in the Go repository. Define how asset versions are added and incremented, how old records remain available while primary-key pointers change, and how queries order by descending version. Done means historical asset balances can be resolved for a block hash using the specified joins.

Written by the indexing model from the issue text.