rpc: maxoffset check does not activate since PR #34197

[knz]

Found while investigating #84017.

Describe the problem

The MaxOffset check during RPC heartbeats depends on the Offset field being set in-between PingRequest heartbeats by the client.

(from pkg/rpc/context.go)

In PR #34197, we changed the code such that the Offset field is never set:

Meanwhile, we also do not currently (neither then, nor now) have a unit test that verifies that the MaxOffset check runs on hearbeats, so this regression could not be caught then.

Expected Behavior

The hearbeat check includes the Offset field again, and there's a test to confirm that.

Jira issue: CRDB-17418

[knz]

@bdarnell and @ajwerner had a look. They independently observed that we still get "half" of the checks:

There's an update/check both:

• when a node gets pinged by a remote node using the remote-provided .Offset,
• when receiving a ping response, using an estimate of the roundtrip latency between PingRequest-PingResponse.

The bug above only invalidates the first check. The second check still occurs.
Since all nodes are both client to another node, and server for the same other node, we get a check both ways on all pairs of nodes.