Update prefetch count for a receiver (#441)

Author: sjkwak

azure-eventhubs/src/main/java/com/microsoft/azure/eventhubs/PartitionReceiver.java

@@ -26,7 +26,7 @@ public interface PartitionReceiver {
 
     int MINIMUM_PREFETCH_COUNT = 1;
     int DEFAULT_PREFETCH_COUNT = 500;
-    int MAXIMUM_PREFETCH_COUNT = 2000;
+    int MAXIMUM_PREFETCH_COUNT = 8000;
 
     long NULL_EPOCH = 0;
 
@@ -141,4 +141,4 @@ default Iterable<EventData> receiveSync(final int maxEventCount) throws EventHub
     CompletableFuture<Void> close();
 
     void closeSync() throws EventHubException;
-}
+}