[CORD-2838][CORD-2833] Revisit McastHandler and handle shortest paths with pair links

Includes also a refactoring of the path computation

Change-Id: Iff63780a3bb3e895e55c52211290c19d993e1905
diff --git a/apps/segmentrouting/BUCK b/apps/segmentrouting/BUCK
index f70eb88..c03ace3 100644
--- a/apps/segmentrouting/BUCK
+++ b/apps/segmentrouting/BUCK
@@ -9,5 +9,5 @@
     url = 'http://onosproject.org',
     included_bundles = BUNDLES,
     description = 'Segment routing application.',
-    required_apps = [ 'org.onosproject.route-service' ],
+    required_apps = [ 'org.onosproject.route-service', 'org.onosproject.mcast' ],
 )