sql: cancel duplicate stats requests once unblocked by other long-running stats requests

When a gateway node makes the probabilistic decision to request stats for a table, it will schedule a retry for that request if there is already another stats job running. The retry is scheduled here by adding a `mutation` with `rowsAffected=math.MaxInt32` to the `mutations` channel:

https://github.com/cockroachdb/cockroach/blob/3794f0e7511765473fe38efb612a46cd8fb564aa/pkg/sql/stats/automatic_stats.go#L808-L813

That retry is always guaranteed to eventually collect stats successfully, by this conditional:

https://github.com/cockroachdb/cockroach/blob/3794f0e7511765473fe38efb612a46cd8fb564aa/pkg/sql/stats/automatic_stats.go#L789-L792

Assume there is a long-running stats job that blocks stats requests on `table1`. If the job runs long enough, it is possible, even probably in some cases, that more than on gateway node will request stats on another table, `table2`. Once the long-running stats job for `table1` completes, each node requesting stats on `table2` will race to start a new job, and although one will win, the other nodes' requests will not be canceled once stats have been collected on `table2`. As a result, stats will be collected for every node that decided to request stats, even if stats were just collected.

I could imagine situations in which many stats would be collected for the same table, one right after the other.

To fix this, I believe we need the retry mechanism to consider whether stats for the table were recently collected. Or perhaps there is another suitable heuristic.

Jira issue: CRDB-38411