Downgrade the tx isolation level of poll message ack flow (#2615)

This might help alleviate DB transaction contention on the PollMessage table. A
lower transaction isolation level is safe because acking a poll message is
idempotent: there are only two things it does, either delete a poll message or
take a recurring one from the past and set it to be a year in the future from
the date in the past. Both of these operations will always yield the same final
result even if executed multiple times simultaneously for some reason.

Author: CydeWeys

core/src/main/java/google/registry/flows/poll/PollAckFlow.java

@@ -37,6 +37,8 @@
 import google.registry.model.poll.PollMessage;
 import google.registry.model.poll.PollMessageExternalKeyConverter;
 import google.registry.model.poll.PollMessageExternalKeyConverter.PollMessageExternalKeyParseException;
+import google.registry.persistence.IsolationLevel;
+import google.registry.persistence.PersistenceModule.TransactionIsolationLevel;
 import google.registry.persistence.VKey;
 import java.util.Optional;
 import javax.inject.Inject;
@@ -55,6 +57,7 @@
  * @error {@link PollAckFlow.MissingMessageIdException}
  * @error {@link PollAckFlow.NotAuthorizedToAckMessageException}
  */
+@IsolationLevel(value = TransactionIsolationLevel.TRANSACTION_READ_COMMITTED)
 public final class PollAckFlow implements MutatingFlow {
 
   @Inject ExtensionManager extensionManager;