fix: Thread safe UpdateBuilder

google-cloud-firestore/src/main/java/com/google/cloud/firestore/UpdateBuilder.java

@@ -36,6 +36,7 @@
 import java.util.Map.Entry;
 import java.util.SortedSet;
 import java.util.TreeSet;
+import java.util.concurrent.CopyOnWriteArrayList;
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 
@@ -62,7 +63,7 @@ public String toString() {
 
   final FirestoreImpl firestore;
 
-  private final List<WriteOperation> writes = new ArrayList<>();
+  private final List<WriteOperation> writes = new CopyOnWriteArrayList<>();
 
   protected boolean committed;