Integrate transaction persistence into JpaTM (#717)

* Integrate transaction persistence into JpaTM

Store the serialized transaction whenever we commit from the JPA transaction
manager.  This change also adds:

-   The Transaction table.
-   The TransactionEntity which is stored in it.
-   Changes to the test infrastructure to register the TransactionEntity for
    tests where we don't load the nomulus schema.
-   A new configuration variable to allow us to turn the transaction
    persistence functionality on and off (default is "off").

* Changes for review.

* Incremented sequence number of flyway file

db/src/main/resources/sql/flyway/V42__add_txn_table.sql

@@ -0,0 +1,19 @@
+-- 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.
+
+create table "Transaction" (
+   id  bigserial not null,
+    contents bytea,
+    primary key (id)
+);