Fix cross-TX header introduction glitch

Commit e79ba5fe88 introduced an off-by-one
error which caused CX-enabled header to be introduced one epoch later
than expected, i.e. at CrossTxEpoch + 1, where it should be
CrossTxEpoch.  This was caused by a vague, confusing naming of
IsCrossTx(epoch) function of ChainConfig: The name did not clarify
whether it meant introduction of CX-enabled header and processing of
ingress receipts (which occurs at CrossTxEpoch) or acceptance of
cross-shard transactions (which occurs at CrossTxEpoch+1).

Introduce and use params.(*ChainConfig).HasCrossTxFields which handles
the former case.

block/factory/factory.go

@@ -34,7 +34,7 @@ func (f *factory) NewHeader(epoch *big.Int) *block.Header {
 		impl = v3.NewHeader()
 	case f.chainConfig.IsCrossLink(epoch):
 		impl = v2.NewHeader()
-	case f.chainConfig.IsCrossTx(epoch):
+	case f.chainConfig.HasCrossTxFields(epoch):
 		impl = v1.NewHeader()
 	default:
 		impl = v0.NewHeader()

internal/params/config.go

@@ -139,21 +139,28 @@ func (c *ChainConfig) IsEIP155(epoch *big.Int) bool {
 	return isForked(c.EIP155Epoch, epoch)
 }
 
-// IsCrossTx returns whether cross-shard transaction is enabled in the given
+// IsCrossTx returns whether cross-shard transaction is accepted in the given
 // epoch.
 //
 // Note that this is different from comparing epoch against CrossTxEpoch.
-// Cross-shard transaction is enabled from CrossTxEpoch+1 and on, in order to
+// Cross-shard transaction is accepted from CrossTxEpoch+1 and on, in order to
 // allow for all shards to roll into CrossTxEpoch and become able to handle
 // ingress receipts.  In other words, cross-shard transaction fields are
-// introduced at CrossTxEpoch, but these fields are not used until
-// CrossTxEpoch+1, when cross-shard transactions are actually accepted by the
-// network.
+// introduced and ingress receipts are processed at CrossTxEpoch, but the shard
+// does not accept cross-shard transactions from clients until CrossTxEpoch+1.
+//
+// TODO ek – rename to AcceptsCrossTx to clarify
 func (c *ChainConfig) IsCrossTx(epoch *big.Int) bool {
 	crossTxEpoch := new(big.Int).Add(c.CrossTxEpoch, common.Big1)
 	return isForked(crossTxEpoch, epoch)
 }
 
+// HasCrossTxFields returns whether blocks in the given epoch includes
+// cross-shard transaction fields.
+func (c *ChainConfig) HasCrossTxFields(epoch *big.Int) bool {
+	return isForked(c.CrossTxEpoch, epoch)
+}
+
 // IsStaking determines whether it is staking epoch
 func (c *ChainConfig) IsStaking(epoch *big.Int) bool {
 	return isForked(c.StakingEpoch, epoch)