Fix sporadic SQL Snapshot failure (#1571)

* Fix sporadic SQL Snapshot failure

The Postgresql set-snapshot statement (called in
JpaTransactionManager.setDatabaseSnapshot() method) must be the first
statement in the SQL transaction.

Currenty the JpaTransaction.transact() method may insert a query for
DatabaseMigrationStateSchedule before the user query when the cache is
empty or the cached value expires.

This PR proactively preloads the cache in RegistryJpaIO to prevent cache
loading  inside the transaction.

This PR also changes some DatabaseSnapshotTest tests to be retrying, in
case they run just after the cache expires. (This has happened before in
CI).

core/src/test/java/google/registry/beam/common/DatabaseSnapshotTest.java

@@ -37,6 +37,7 @@ import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.RegisterExtension;
+import org.junitpioneer.jupiter.RetryingTest;
 import org.testcontainers.containers.PostgreSQLContainer;
 import org.testcontainers.junit.jupiter.Container;
 import org.testcontainers.junit.jupiter.Testcontainers;
@@ -106,7 +107,7 @@ public class DatabaseSnapshotTest {
     try (DatabaseSnapshot databaseSnapshot = DatabaseSnapshot.createSnapshot()) {}
   }
 
-  @Test
+  @RetryingTest(2) // TODO(b/226945065) revert to @Test when DatbaseMigrationSchedule is removed.
   void readSnapshot() {
     try (DatabaseSnapshot databaseSnapshot = DatabaseSnapshot.createSnapshot()) {
       Registry snapshotRegistry =
@@ -120,7 +121,7 @@ public class DatabaseSnapshotTest {
     }
   }
 
-  @Test
+  @RetryingTest(2) // TODO(b/226945065) revert to @Test when DatbaseMigrationSchedule is removed.
   void readSnapshot_withSubsequentChange() {
     try (DatabaseSnapshot databaseSnapshot = DatabaseSnapshot.createSnapshot()) {
       Registry updated =