MastershipService.relinquishRole returns CompletableFuture + Block deviceDiconnected until role relinquish is complete

Change-Id: I081df48fc05fdca2e452a937a093d5caa16091ed
diff --git a/providers/lldp/src/test/java/org/onosproject/provider/lldp/impl/LLDPLinkProviderTest.java b/providers/lldp/src/test/java/org/onosproject/provider/lldp/impl/LLDPLinkProviderTest.java
index 4395914..f6627af 100644
--- a/providers/lldp/src/test/java/org/onosproject/provider/lldp/impl/LLDPLinkProviderTest.java
+++ b/providers/lldp/src/test/java/org/onosproject/provider/lldp/impl/LLDPLinkProviderTest.java
@@ -502,8 +502,8 @@
         }
 
         @Override
-        public void relinquishMastership(DeviceId deviceId) {
-
+        public CompletableFuture<Void> relinquishMastership(DeviceId deviceId) {
+            return null;
         }
 
         @Override