Use keyspace metadata in BasicLoadBalancingPolicy#getReplicas

Modifies the getReplicas method to do the tablet lookup if and only if the
keyspace metadata indicates that it's a tablets-based keyspace. Otherwise refer
to the token map.
Previous behavior was to try tablet map lookup first regardless of the
keyspace configuration.

core/src/main/java/com/datastax/oss/driver/internal/core/loadbalancing/BasicLoadBalancingPolicy.java

@@ -37,6 +37,7 @@
 import com.datastax.oss.driver.api.core.metadata.Tablet;
 import com.datastax.oss.driver.api.core.metadata.TabletMap;
 import com.datastax.oss.driver.api.core.metadata.TokenMap;
+import com.datastax.oss.driver.api.core.metadata.schema.KeyspaceMetadata;
 import com.datastax.oss.driver.api.core.metadata.token.Partitioner;
 import com.datastax.oss.driver.api.core.metadata.token.Token;
 import com.datastax.oss.driver.api.core.session.Request;
@@ -328,7 +329,12 @@ protected Set<Node> getReplicas(@Nullable Request request, @Nullable Session ses
       return Collections.emptySet();
     }
 
-    if (table != null) {
+    Optional<KeyspaceMetadata> ksMetadata =
+        context.getMetadataManager().getMetadata().getKeyspace(keyspace);
+    if (ksMetadata.isPresent() && ksMetadata.get().isUsingTablets()) {
+      if (table == null) {
+        return Collections.emptySet();
+      }
       if (token == null) {
         if (partitioner != null) {
           token = partitioner.hash(key);
@@ -338,12 +344,10 @@ protected Set<Node> getReplicas(@Nullable Request request, @Nullable Session ses
         Tablet targetTablet =
             tabletMap.getTablet(keyspace, table, ((TokenLong64) token).getValue());
         if (targetTablet != null) {
-          Set<Node> replicas = targetTablet.getReplicaNodes();
-          if (!replicas.isEmpty()) {
-            return replicas;
-          }
+          return targetTablet.getReplicaNodes();
         }
       }
+      return Collections.emptySet();
     }
 
     if (!maybeTokenMap.isPresent()) {