fix: mempool: check messages against the next block's height

Previously, we checked message gas/validity with the previous block's
height. This doesn't affect consensus, but will help us avoid adding
messages to the message pool that shouldn't be there.

chain/messagepool/check.go

@@ -106,7 +106,7 @@ func (mp *MessagePool) checkMessages(ctx context.Context, msgs []*types.Message,
 	curTs := mp.curTs
 	mp.curTsLk.Unlock()
 
-	epoch := curTs.Height()
+	epoch := curTs.Height() + 1
 
 	var baseFee big.Int
 	if len(curTs.Blocks()) > 0 {

chain/messagepool/messagepool.go

@@ -628,7 +628,7 @@ func (mp *MessagePool) addLocal(ctx context.Context, m *types.SignedMessage) err
 // For non local messages, if the message cannot be included in the next 20 blocks it returns
 // a (soft) validation error.
 func (mp *MessagePool) verifyMsgBeforeAdd(m *types.SignedMessage, curTs *types.TipSet, local bool) (bool, error) {
-	epoch := curTs.Height()
+	epoch := curTs.Height() + 1
 	minGas := vm.PricelistByEpoch(epoch).OnChainMessage(m.ChainLength())
 
 	if err := m.VMMessage().ValidForBlockInclusion(minGas.Total(), build.NewestNetworkVersion); err != nil {