Allow replicateToDatastore to skip gaps (#1539)

* Allow replicateToDatastore to skip gaps

As it turns out, gaps in the transaction id sequence number are expected
because rollbacks do not rollback sequence numbers.

To deal with this, stop checking these.

This change is not adequate in and of itself, as it is possible for a gap to
be introduced if two transactions are committed out of order of their sequence
number.  We are currently discussing several strategies to mitigate this.

* Remove println, add a record verification

core/src/main/java/google/registry/model/replay/ReplicateToDatastoreAction.java

@@ -128,13 +128,10 @@ public class ReplicateToDatastoreAction implements Runnable {
                 LastSqlTransaction lastSqlTxn = LastSqlTransaction.load();
                 long nextTxnId = lastSqlTxn.getTransactionId() + 1;
                 if (nextTxnId < txnEntity.getId()) {
-                  // We're missing a transaction.  This is bad.  Transaction ids are supposed to
-                  // increase monotonically, so we abort rather than applying anything out of
-                  // order.
-                  throw new IllegalStateException(
-                      String.format(
-                          "Missing transaction: last txn id = %s, next available txn = %s",
-                          nextTxnId - 1, txnEntity.getId()));
+                  // Missing transaction id.  This can happen normally.  If a transaction gets
+                  // rolled back, the sequence counter doesn't.
+                  logger.atWarning().log(
+                      "Ignoring transaction %s, which does not exist.", nextTxnId);
                 } else if (nextTxnId > txnEntity.getId()) {
                   // We've already replayed this transaction.  This shouldn't happen, as GAE cron
                   // is supposed to avoid overruns and this action shouldn't be executed from any

core/src/main/java/google/registry/persistence/transaction/TransactionEntity.java

@@ -14,6 +14,7 @@
 
 package google.registry.persistence.transaction;
 
+import com.google.common.annotations.VisibleForTesting;
 import google.registry.model.ImmutableObject;
 import google.registry.model.replay.SqlOnlyEntity;
 import javax.persistence.Entity;
@@ -39,7 +40,8 @@ public class TransactionEntity extends ImmutableObject implements SqlOnlyEntity
 
   TransactionEntity() {}
 
-  TransactionEntity(byte[] contents) {
+  @VisibleForTesting
+  public TransactionEntity(byte[] contents) {
     this.contents = contents;
   }