Assign unused Future return value to a variable

Error Prone plans to make this pattern into a compile error; this avoids
future breakage.

In some of these cases, it's not obvious that ignoring the return value
is the correct thing to do; the hope is that the construct is ugly
enough to make the author think twice. In many cases though, it's clear
that a locally-created Runnable or Callable already does its own error
handling, so we can leave a comment alongside the ignored return value.

Eventually many of these should be audited again, and we can replace
some of the @SuppressWarnings with Error Prone's @CanIgnoreReturnValue.
That much is left for future cleanup.

Change-Id: Ia989214d85e0d6c387e388a77178e0b5c4bf2498

gerrit-cache-h2/src/main/java/com/google/gerrit/server/cache/h2/H2CacheFactory.java

@@ -39,6 +39,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 import org.eclipse.jgit.lib.Config;
@@ -116,7 +117,9 @@ class H2CacheFactory implements PersistentCacheFactory, LifecycleListener {
     if (executor != null) {
       for (final H2CacheImpl<?, ?> cache : caches) {
         executor.execute(cache::start);
-        cleanup.schedule(() -> cache.prune(cleanup), 30, TimeUnit.SECONDS);
+        @SuppressWarnings("unused")
+        Future<?> possiblyIgnoredError =
+            cleanup.schedule(() -> cache.prune(cleanup), 30, TimeUnit.SECONDS);
       }
     }
   }

gerrit-cache-h2/src/main/java/com/google/gerrit/server/cache/h2/H2CacheImpl.java

@@ -46,6 +46,7 @@ import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Executor;
+import java.util.concurrent.Future;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicLong;
@@ -196,7 +197,9 @@ public class H2CacheImpl<K, V> extends AbstractLoadingCache<K, V> implements Per
     cal.add(Calendar.DAY_OF_MONTH, 1);
 
     long delay = cal.getTimeInMillis() - TimeUtil.nowMs();
-    service.schedule(() -> prune(service), delay, TimeUnit.MILLISECONDS);
+    @SuppressWarnings("unused")
+    Future<?> possiblyIgnoredError =
+        service.schedule(() -> prune(service), delay, TimeUnit.MILLISECONDS);
   }
 
   static class ValueHolder<V> {