Fix nasty race condition in the MeterStore

This situation is very specific of the EC maps which
can receive updates from the peers immedietaly due
to the bootstrap process.

Change-Id: I6b6087fbf907d8a6fb0bb8f807fe210e0f1ab571
diff --git a/core/store/dist/src/main/java/org/onosproject/store/meter/impl/DistributedMeterStore.java b/core/store/dist/src/main/java/org/onosproject/store/meter/impl/DistributedMeterStore.java
index b94375c..30da74d 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/meter/impl/DistributedMeterStore.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/meter/impl/DistributedMeterStore.java
@@ -114,7 +114,8 @@
 
     // Meters id related objects
     private static final String AVAILABLEMETERIDSTORE = "onos-meters-available-store";
-    protected ConcurrentMap<MeterTableKey, DistributedSet<MeterKey>> availableMeterIds;
+    protected final ConcurrentMap<MeterTableKey, DistributedSet<MeterKey>> availableMeterIds =
+            new ConcurrentHashMap<>();
     private static final String METERIDSTORE = "onos-meters-id-store";
     private AtomicCounterMap<MeterTableKey> meterIdGenerators;
 
@@ -177,8 +178,6 @@
                 .withSerializer(APP_KRYO_BUILDER).build();
         metersFeatures.addListener(featuresMapListener);
 
-        availableMeterIds = new ConcurrentHashMap<>();
-
         meterIdGenerators = storageService.<MeterTableKey>atomicCounterMapBuilder()
                 .withName(METERIDSTORE)
                 .withSerializer(Serializer.using(KryoNamespaces.API,