Panic if InitiateHeartbeat exhausts retries to avoid looping infinitely (#1594)

* Panic if InitiateHeartbeat exhausts retries to avoid looping infinitely.

Based on experience, if the writer database fails inbeetween the copy & cutover stages (e.g. during cutover pause), the heartbeat writes will fail and stop,
then leading to throttled state and an infinite loop of throttler.shouldThrottle().

Since this state is irrecoverable, make the heartbeat writer panic if retries are exhausted, so that the migration can fail and be restarted later.

* Add sysbench localtest (#1590)

* add sysbench localtest

* fix table name

* Apply suggestion from @Copilot

Co-authored-by: Copilot <[email protected]>

* ensure cleanup

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Co-authored-by: Copilot <[email protected]>

* add toxiproxy option for localtests (#1591)

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Co-authored-by: Jan Grodowski <[email protected]>

Author: meiji163

go/logic/applier.go

@@ -575,6 +575,7 @@ func (this *Applier) InitiateHeartbeat() {
 			continue
 		}
 		if err := injectHeartbeat(); err != nil {
+			this.migrationContext.PanicAbort <- fmt.Errorf("injectHeartbeat writing failed %d times, last error: %w", numSuccessiveFailures, err)
 			return
 		}
 	}