Avoid ConcurrentModificationException by using SynchronizedMultimap

Change-Id: Ic49ac1d63d600e685d150fa4e94132a403189c84
diff --git a/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp4HandlerImpl.java b/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp4HandlerImpl.java
index bc37ca8..d2c146b 100644
--- a/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp4HandlerImpl.java
+++ b/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp4HandlerImpl.java
@@ -20,6 +20,7 @@
 import com.google.common.collect.HashMultimap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Multimap;
+import com.google.common.collect.Multimaps;
 import com.google.common.collect.Sets;
 import org.apache.felix.scr.annotations.Activate;
 import org.apache.felix.scr.annotations.Component;
@@ -167,7 +168,7 @@
 
     protected HostProviderService providerService;
     protected ApplicationId appId;
-    protected Multimap<DeviceId, VlanId> ignoredVlans = HashMultimap.create();
+    protected Multimap<DeviceId, VlanId> ignoredVlans = Multimaps.synchronizedMultimap(HashMultimap.create());
     private InternalHostListener hostListener = new InternalHostListener();
 
     private List<DhcpServerInfo> defaultServerInfoList = new CopyOnWriteArrayList<>();
diff --git a/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp6HandlerImpl.java b/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp6HandlerImpl.java
index e5f52bf..c4ae632 100644
--- a/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp6HandlerImpl.java
+++ b/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp6HandlerImpl.java
@@ -19,6 +19,7 @@
 
 import com.google.common.collect.HashMultimap;
 import com.google.common.collect.Multimap;
+import com.google.common.collect.Multimaps;
 import org.apache.felix.scr.annotations.Activate;
 import org.apache.felix.scr.annotations.Deactivate;
 import com.google.common.collect.Sets;
@@ -168,7 +169,7 @@
 
     protected HostProviderService providerService;
     protected ApplicationId appId;
-    protected Multimap<DeviceId, VlanId> ignoredVlans = HashMultimap.create();
+    protected Multimap<DeviceId, VlanId> ignoredVlans = Multimaps.synchronizedMultimap(HashMultimap.create());
     private InternalHostListener hostListener = new InternalHostListener();
 
     private List<DhcpServerInfo> defaultServerInfoList = new CopyOnWriteArrayList<>();