cleanup: prevent leaks from time.After

This PR replaces use of time.After with a safe helper function
that creates a time.Timer to use instead. The new function returns
both a time.Timer and a Stop function that the caller must handle.

Unlike time.NewTimer, the helper function does not panic if the duration
set is <= 0.

client/allocrunner/taskrunner/task_runner.go

@@ -527,6 +527,9 @@ func (tr *TaskRunner) Run() {
 		return
 	}
 
+	timer, stop := helper.NewSafeTimer(0) // timer duration calculated JIT
+	defer stop()
+
 MAIN:
 	for !tr.shouldShutdown() {
 		select {
@@ -612,9 +615,11 @@ MAIN:
 			break MAIN
 		}
 
+		timer.Reset(restartDelay)
+
 		// Actually restart by sleeping and also watching for destroy events
 		select {
-		case <-time.After(restartDelay):
+		case <-timer.C:
 		case <-tr.killCtx.Done():
 			tr.logger.Trace("task killed between restarts", "delay", restartDelay)
 			break MAIN