private void handleUnavailableHosts(final List<Host> unavailableHosts) {
            // start the re-initialization attempt for each of the unavailable hosts through Host.makeUnavailable().
            for (Host host : unavailableHosts) {
                final CompletableFuture<Void> f = CompletableFuture.runAsync(
                        () -> host.makeUnavailable(this::tryReInitializeHost), cluster.hostScheduler());
                f.exceptionally(t -> {
                    logger.error("", (t.getCause() == null) ? t : t.getCause());
                    return null;
                });
            }
        }
Closes all the connection pools on all hosts.
gremlin-driver/src/main/java/org/apache/tinkerpop/gremlin/driver/Client.java