Replace Stream.concat with Streams.concat

Stream.concat only accepts 2 parameters. Streams.concat on the other hand
accepts any number of parameters.

Moving to Streams.concat for all uses (2 or more) makes sense for uniformity
and convenience reasons.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=179716648

java/google/registry/tools/server/KillAllCommitLogsAction.java

@@ -25,6 +25,7 @@ import com.google.appengine.tools.mapreduce.Mapper;
 import com.google.appengine.tools.mapreduce.inputs.InMemoryInput;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Lists;
+import com.google.common.collect.Streams;
 import com.googlecode.objectify.Key;
 import google.registry.config.RegistryEnvironment;
 import google.registry.mapreduce.MapreduceRunner;
@@ -66,7 +67,7 @@ public class KillAllCommitLogsAction implements Runnable {
     Input<Key<?>> input =
         new InMemoryInput<>(
             Lists.partition(
-                Stream.concat(
+                Streams.concat(
                         Stream.of(CommitLogCheckpointRoot.getKey()),
                         CommitLogBucket.getAllBucketKeys().stream())
                     .collect(toImmutableList()),