Allow Attached rules to be flagged 'light' (#580)

Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

ethereum/core/src/main/java/tech/pegasys/pantheon/ethereum/mainnet/AttachedBlockHeaderValidationRule.java

@@ -26,4 +26,8 @@ public interface AttachedBlockHeaderValidationRule<C> {
    * @return {@code true} if valid; otherwise {@code false}
    */
   boolean validate(BlockHeader header, BlockHeader parent, ProtocolContext<C> protocolContext);
+
+  default boolean includeInLightValidation() {
+    return true;
+  }
 }

ethereum/core/src/main/java/tech/pegasys/pantheon/ethereum/mainnet/BlockHeaderValidator.java

@@ -120,7 +120,7 @@ public class BlockHeaderValidator<C> {
     private final List<Rule<C>> rules = new ArrayList<>();
 
     public Builder<C> addRule(final AttachedBlockHeaderValidationRule<C> rule) {
-      this.rules.add(new Rule<>(false, rule, true));
+      this.rules.add(new Rule<>(false, rule, rule.includeInLightValidation()));
       return this;
     }