4.x: Enhance TabletMap API

[dkropachev]

Move following code under TabletMap umbrella:

    Optional<KeyspaceMetadata> ksMetadata =
        context.getMetadataManager().getMetadata().getKeyspace(keyspace);
    if (ksMetadata.isPresent() && ksMetadata.get().isUsingTablets()) {
      if (table == null) {
        return Collections.emptySet();
      }
      if (token instanceof TokenLong64) {
        Tablet targetTablet =
            tabletMap.getTablet(keyspace, table, ((TokenLong64) token).getValue());
        if (targetTablet != null) {
          return targetTablet.getReplicaNodes();
        }
      }
      return Collections.emptySet();
    }