Persist in-memory replication stats to disk (#15594)

to avoid relying on scanner-calculated replication metrics.
This will improve the accuracy of the replication stats reported.

This PR also adds on to #15556 by handing replication
traffic that could not be queued by available workers to the 
MRF queue so that entries in `PENDING` status are healed faster.

cmd/signals.go

@@ -78,9 +78,11 @@ func handleSignals() {
 		case <-globalHTTPServerErrorCh:
 			exit(stopProcess())
 		case osSignal := <-globalOSSignalCh:
+			globalReplicationPool.SaveState(context.Background())
 			logger.Info("Exiting on signal: %s", strings.ToUpper(osSignal.String()))
 			exit(stopProcess())
 		case signal := <-globalServiceSignalCh:
+			globalReplicationPool.SaveState(context.Background())
 			switch signal {
 			case serviceRestart:
 				logger.Info("Restarting on service signal")