Some class of constraints should not be used for link cost evaluation.

ONOS-6021

Current ConnectivityIntentCompiler simply picks first non-negative cost as Link cost value.

Some class of constraints are used to express Path viability or IntentCompiler's behavior.
Those constraints tend to returns fixed arbitrary non-negative link cost, which probably is not the best option to be used as Link cost during path computation.

This patch will:
- Introduce base class for constraints which should not influence Link cost.
- Introduce base class for constraints which should not influence Link cost or Path viability.
- Exclude above classes from link cost computation in ConnectivityIntentCompiler

MarkerConstraint
 base class for Constraints,
 which is not meant to influence Link cost or Path viability.

PathViablityConstraint
 base class for Constraints,
 which is not meant to influence Link cost.

Change-Id: Ice8b83a18cfe3bf5a68c25a853667bfaedb2b1a1
diff --git a/core/api/src/main/java/org/onosproject/net/intent/constraint/ProtectionConstraint.java b/core/api/src/main/java/org/onosproject/net/intent/constraint/ProtectionConstraint.java
index e4dce01..bceebcd 100644
--- a/core/api/src/main/java/org/onosproject/net/intent/constraint/ProtectionConstraint.java
+++ b/core/api/src/main/java/org/onosproject/net/intent/constraint/ProtectionConstraint.java
@@ -17,32 +17,17 @@
 package org.onosproject.net.intent.constraint;
 
 import com.google.common.annotations.Beta;
-import org.onosproject.net.Link;
-import org.onosproject.net.Path;
-import org.onosproject.net.intent.Constraint;
 import org.onosproject.net.intent.Intent;
 import org.onosproject.net.intent.PointToPointIntent;
-import org.onosproject.net.intent.ResourceContext;
 
 /**
  * Constraint that determines whether to employ path protection.
  */
 @Beta
-public class ProtectionConstraint implements Constraint {
+public final class ProtectionConstraint extends MarkerConstraint {
+
     private static final ProtectionConstraint PROTECTION_CONSTRAINT = new ProtectionConstraint();
 
-    // doesn't use LinkResourceService
-    @Override
-    public double cost(Link link, ResourceContext context) {
-        return 1;
-    }
-
-    // doesn't use LinkResourceService
-    @Override
-    public boolean validate(Path path, ResourceContext context) {
-        return true;
-    }
-
     /**
      * Determines whether to utilize path protection for the given intent.
      *