Revert "NoteDbUpdateManager: Remove unused flush method"

This reverts commit 4004a68609.

Reason for revert: There is an internal caller for flush.

Change-Id: I193dcc12f9e89178489a0b19c111dded005cf5eb

gerrit-server/src/main/java/com/google/gerrit/server/notedb/NoteDbUpdateManager.java

@@ -455,6 +455,15 @@ public class NoteDbUpdateManager implements AutoCloseable {
     return draftIds;
   }
 
+  public void flush() throws IOException {
+    if (changeRepo != null) {
+      changeRepo.flush();
+    }
+    if (allUsersRepo != null) {
+      allUsersRepo.flush();
+    }
+  }
+
   @Nullable
   public BatchRefUpdate execute() throws OrmException, IOException {
     return execute(false);