ledger: store balance to avoid full table scan
ledger: store balance to avoid full table scan

Gives the plan

root@:26257/ledger> explain SELECT causality_id, balance FROM accounts WHERE account_id = x ORDER BY causality_id DESC LIMIT 1;
+-------+---------+-------+-----------------------------------------------+
| Level |  Type   | Field |                  Description                  |
+-------+---------+-------+-----------------------------------------------+
|     0 | limit   |       |                                               |
|     1 | render  |       |                                               |
|     2 | revscan |       |                                               |
|     2 |         | table | accounts@accounts_account_id_causality_id_key |
|     2 |         | spans | /"x"-/"x\x00"                                 |
|     2 |         | limit |                                             1 |
+-------+---------+-------+-----------------------------------------------+

where previously a full table scan was necessary to retrieve `balance`, which
was not indexed.

cockroachdb/examples-go,cockroachdb/examples-go