fix: block close() on the termination of all RPC channels (#512)

Author: schmidt-sebastian

google-cloud-firestore/src/main/java/com/google/cloud/firestore/spi/v1/GrpcFirestoreRpc.java

@@ -66,6 +66,7 @@
 import java.io.IOException;
 import java.util.Collections;
 import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
 
 /**
  * GRPC client implementation of the Firestore RPC methods. Exposes FirestoreOptions on top of the
@@ -152,6 +153,9 @@ public void close() throws Exception {
     for (BackgroundResource resource : clientContext.getBackgroundResources()) {
       resource.close();
     }
+    for (BackgroundResource resource : clientContext.getBackgroundResources()) {
+      resource.awaitTermination(1, TimeUnit.SECONDS);
+    }
     executorFactory.release(executor);
   }