Avoid contention over the RefreshDnsRequest table (#2428)

* Avoid contention over the RefreshDnsRequest table

This table can be small at times, when PSQL may use table scan in
queries by keys. At the SERIALIZABLE isolation level, updates to
unrelated rows may conflict due to this `optimization`.

Lower the isolation level to repeatable read.

* Code review

Author: weiminyu

Diff for: core/src/main/java/google/registry/dns/DnsUtils.java

@@ -15,6 +15,7 @@
 package google.registry.dns;
 
 import static com.google.common.collect.ImmutableList.toImmutableList;
+import static google.registry.persistence.PersistenceModule.TransactionIsolationLevel.TRANSACTION_REPEATABLE_READ;
 import static google.registry.persistence.transaction.TransactionManagerFactory.tm;
 
 import com.google.common.collect.ImmutableCollection;
@@ -98,7 +99,9 @@ public static void requestHostDnsRefresh(ImmutableCollection<String> hostNames)
    */
   public static ImmutableList<DnsRefreshRequest> readAndUpdateRequestsWithLatestProcessTime(
       String tld, Duration cooldown, int batchSize) {
+    // It is critical that below query use repeatable-read. See b/337894387.
     return tm().transact(
+            TRANSACTION_REPEATABLE_READ,
             () -> {
               DateTime transactionTime = tm().getTransactionTime();
               ImmutableList<DnsRefreshRequest> requests =
@@ -131,7 +134,9 @@ public static ImmutableList<DnsRefreshRequest> readAndUpdateRequestsWithLatestPr
    * error because all we care about is that it no longer exists after the method runs.
    */
   public static void deleteRequests(Collection<DnsRefreshRequest> requests) {
+    // It is critical that below query use repeatable-read. See b/337894387.
     tm().transact(
+            TRANSACTION_REPEATABLE_READ,
             () ->
                 tm().delete(
                         requests.stream()