[enhancement] store balances at block heights

[joel-u410]

Currently the balances table just contains one balance row per (owner, token) pair, and it is updated continuously as balances change on-chain. I would like to propose changing it to record balance history by doing the following:

• Rename balances to balance_changes and add a column for height.
• When querying balances from the node via RPC, be specific about which height is requested (see this PR for an update to namada_sdk that makes this possible).
• Instead of updating rows in balances, insert a new row to balance_changes whenever a new balance is queried.
• (if needed) Introduce a new view named balances which takes the place of the current balances table and provides the most recent balance by height for each (owner, token).

I am working on this in my fork and expect to have a PR soon that implements these changes.