Change return type of TabletMap#getReplicas() to Set<Host>

Having this method return set of hosts instead saves one `Map#get()` query
per host.
In theory, previously it was also possible that between `TabletMap#getReplicas()`
returning and `Metadata#getReplicas()` translating uuids to hosts, some hosts could
become invalid, making the translation represent them as nulls.

driver-core/src/main/java/com/datastax/driver/core/Metadata.java

@@ -44,7 +44,6 @@
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
-import java.util.stream.Collectors;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -570,9 +569,9 @@ public Set<Host> getReplicas(
       if (keyspace != null && table != null) {
         Token token = partitioner.hash(partitionKey);
         assert (token instanceof Token.TokenLong64);
-        Set<UUID> hostUuids = tabletMap.getReplicas(keyspace, table, (long) token.getValue());
-        if (!hostUuids.isEmpty()) {
-          return hostUuids.stream().map(this::getHost).collect(Collectors.toSet());
+        Set<Host> replicas = tabletMap.getReplicas(keyspace, table, (long) token.getValue());
+        if (!replicas.isEmpty()) {
+          return replicas;
         }
       }
       // Fall back to tokenMap

driver-core/src/main/java/com/datastax/driver/core/TabletMap.java

@@ -64,9 +64,9 @@ public Map<KeyspaceTableNamePair, TabletSet> getMapping() {
    * @param keyspace the keyspace that table is in
    * @param table the table name
    * @param token the token to look for
-   * @return Set of host UUIDS that do have a tablet for given token for a given table.
+   * @return Set of Host instances that do have a tablet for the given token and table combination.
    */
-  public Set<UUID> getReplicas(String keyspace, String table, long token) {
+  public Set<Host> getReplicas(String keyspace, String table, long token) {
     TabletMap.KeyspaceTableNamePair key = new TabletMap.KeyspaceTableNamePair(keyspace, table);
 
     if (mapping == null) {
@@ -93,18 +93,19 @@ public Set<UUID> getReplicas(String keyspace, String table, long token) {
         return Collections.emptySet();
       }
 
-      HashSet<UUID> uuidSet = new HashSet<>();
+      HashSet<Host> replicaSet = new HashSet<>();
       for (HostShardPair hostShardPair : row.replicas) {
-        if (cluster.metadata.getHost(hostShardPair.getHost()) == null) {
+        Host replica = cluster.metadata.getHost(hostShardPair.getHost());
+        if (replica == null) {
           // We've encountered a stale host. Return an empty set to
           // misroute the request. If misrouted then response will
           // contain up to date tablet information that will be processed.
           return Collections.emptySet();
         } else {
-          uuidSet.add(hostShardPair.getHost());
+          replicaSet.add(replica);
         }
       }
-      return uuidSet;
+      return replicaSet;
     } finally {
       readLock.unlock();
     }

driver-core/src/main/java/com/datastax/driver/core/TabletMapListener.java

@@ -1,5 +1,7 @@
 package com.datastax.driver.core;
 
+// This class does not handle topology changes. Node removal is covered by TabletMap#getReplicas()
+// implementation and tablet overlap is resolved when adding new tablets.
 public class TabletMapListener extends SchemaChangeListenerBase {
   private final TabletMap tabletMap;