Create local storage for topic candidates mapping. This also includes:

 - using Optional in Leadership, and some commenting.
 - using MutableBooleans + compute()

    part of: Device Mastership store on top of LeadershipService
    Reference: ONOS-76

Conflicts:
	core/api/src/main/java/org/onosproject/cluster/LeadershipService.java
	core/store/dist/src/main/java/org/onosproject/store/consistent/impl/DistributedLeadershipManager.java

Change-Id: I7f090abb123cf23bb5126a935a6e72be00f3e3ce
diff --git a/core/store/dist/src/main/java/org/onosproject/store/cluster/impl/HazelcastLeadershipService.java b/core/store/dist/src/main/java/org/onosproject/store/cluster/impl/HazelcastLeadershipService.java
index 10bf6a4..1d918c8 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/cluster/impl/HazelcastLeadershipService.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/cluster/impl/HazelcastLeadershipService.java
@@ -576,7 +576,7 @@
     }
 
     @Override
-    public Map<String, List<NodeId>> getCandidates() {
+    public Map<String, Leadership> getCandidates() {
         return null;
     }