private ServerLocationResponse queryOneLocator(HostAndPort locator,
      ServerLocationRequest request) {
    return queryOneLocatorUsingConnection(locator, request, tcpClient);
  }
key is the InetSocketAddress of the locator. value will be an exception if we have already
found the locator to be dead. value will be null if we last saw it alive.
geode-core/src/main/java/org/apache/geode/cache/client/internal/AutoConnectionSourceImpl.java