[ONOS-7054] Implement prototype of ISSU protocol

Change-Id: Id543c0de9c97b68f977c824cbc987b35d81beb2d
diff --git a/core/api/src/main/java/org/onosproject/cluster/PartitionId.java b/core/api/src/main/java/org/onosproject/cluster/PartitionId.java
index f8b65e7..39852de 100644
--- a/core/api/src/main/java/org/onosproject/cluster/PartitionId.java
+++ b/core/api/src/main/java/org/onosproject/cluster/PartitionId.java
@@ -25,6 +25,11 @@
 public class PartitionId extends Identifier<Integer> implements Comparable<PartitionId> {
 
     /**
+     * The {@code PartitionId} for the shared coordination partition.
+     */
+    public static final PartitionId SHARED = PartitionId.from(0);
+
+    /**
      * Creates a partition identifier from an integer.
      *
      * @param id input integer