Schema_146: Fix FutureReturnValueIgnored warning flagged by error prone

Error prone complaining about non used Future return value:

  Schema_146.java:111: warning: [FutureReturnValueIgnored] Return \
  value of methods returning Future must be checked. Ignoring returned \
  Futures suppresses exceptions thrown from the code that completes \
  the Future.
        pool.submit(() -> processBatch(db, batch, ui));
                   ^
    (see https://errorprone.info/bugpattern/FutureReturnValueIgnored)

Change-Id: Idcfe6d2eb0637eb01aa8224fefa8d24fbdae31e2

java/com/google/gerrit/server/schema/Schema_146.java

@@ -42,6 +42,7 @@ import java.util.Map.Entry;
 import java.util.Set;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.locks.ReentrantLock;
@@ -105,7 +106,12 @@ public class Schema_146 extends SchemaVersion {
         Sets.newHashSet(Iterables.partition(accounts, 500));
     ExecutorService pool = createExecutor(ui);
     try {
-      batches.stream().forEach(batch -> pool.submit(() -> processBatch(db, batch, ui)));
+      batches.stream()
+          .forEach(
+              batch -> {
+                @SuppressWarnings("unused")
+                Future<?> unused = pool.submit(() -> processBatch(db, batch, ui));
+              });
       pool.shutdown();
       pool.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);
     } catch (InterruptedException e) {