jobs: we should make a single node in charge of jobs cleanup

[andreimatei]

In cockroachlabs/support#674 we see gc-jobs queries contending from multiple nodes, and thrashing. It'd be much better if a single node was in charge of running this jobs deletion query, not all the nodes. #57041 makes the situation better by hopefully reducing the thrashing when contention does happen, but still.
I've discussed with @ajwerner that this would be a good time to introduce a general leases table for the cluster, and use it to assign responsibility for such background processes. The leases in the table could be implemented on top of the recent instances/sessions infrastructure (see crdb_internal.sql_liveness_is_alive(session_id)).

Jira issue: CRDB-2877

[blathers-crl]

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[miretskiy]

@ajwerner do you know if we are still susceptible to this? I recall lots of changes that dropped for update clauses, etc...