Fix race condition: broker not scheduling read for active consumer (a…

…pache#3411)

pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentDispatcherSingleActiveConsumer.java

@@ -62,7 +62,7 @@ public final class PersistentDispatcherSingleActiveConsumer extends AbstractDisp
     private int readBatchSize;
     private final Backoff readFailureBackoff = new Backoff(15, TimeUnit.SECONDS, 1, TimeUnit.MINUTES, 0, TimeUnit.MILLISECONDS);
     private final ServiceConfiguration serviceConfig;
-    private ScheduledFuture<?> readOnActiveConsumerTask = null;
+    private volatile ScheduledFuture<?> readOnActiveConsumerTask = null;
 
     private final RedeliveryTracker redeliveryTracker;