Add an extension to verify transaction replay (#857)

* Add an extension to verify transaction replay

Add ReplayExtension, which can be applied to test suites to verify that
transactions committed to datastore can be replayed to SQL.

This introduces a ReplayQueue class, which serves as a stand-in for the
current lack of replay-from-commit-logs.  It also includes replay logic in
TransactionInfo which introduces the concept of "entity class weights."
Entity weighting allows us store and delete objects in an order that is
consistent with the direction of foreign key and deferred foreign key
relationships.  As a general rule, lower weight classes must have no direct or
indirect non-deferred foreign key relationships on higher weight classes.

It is expected that much of this code will change when the final replay
mechanism is implemented.

* Minor fixes:

- Initialize "requestedByRegistrar" to false (it's non-nullable). [reverted
  during rebase: non-nullable was removed in another PR]
- Store test entities (registrar, hosts and contacts) in JPA.

* Make testbed save replay

This changes the replay system to make datastore saves initiated from the
testbed (as opposed to just the tested code) replay when the ReplayExtension
is enabled.  This requires modifications to DatastoreHelper and the
AppEngineExtension that the ReplayExtension can plug into.

This changes also has some necessary fixes to objects that are persisted by
the testbed (such as PremiumList).

db/src/main/resources/sql/flyway/V77__fixes_for_replay.sql

@@ -0,0 +1,15 @@
+-- Copyright 2020 The Nomulus Authors. All Rights Reserved.
+--
+-- Licensed under the Apache License, Version 2.0 (the "License");
+-- you may not use this file except in compliance with the License.
+-- You may obtain a copy of the License at
+--
+--     http://www.apache.org/licenses/LICENSE-2.0
+--
+-- Unless required by applicable law or agreed to in writing, software
+-- distributed under the License is distributed on an "AS IS" BASIS,
+-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+-- See the License for the specific language governing permissions and
+-- limitations under the License.
+
+ALTER TABLE "PremiumList" ALTER COLUMN creation_timestamp DROP NOT NULL;