mempool: disallow more than one issue tx at once

Technically they could conflict for available asset amount, but as they're
very rare (and even can be considered obsolete) we can simplify this check.

pkg/core/mempool/mem_pool_test.go

@@ -177,6 +177,31 @@ func TestMemPoolVerifyClaims(t *testing.T) {
 	require.Error(t, mp.Add(tx3, &FeerStub{}))
 }
 
+func TestMemPoolVerifyIssue(t *testing.T) {
+	mp := NewMemPool(50)
+	tx1 := newIssueTX()
+	require.Equal(t, true, mp.Verify(tx1))
+	require.NoError(t, mp.Add(tx1, &FeerStub{}))
+
+	tx2 := newIssueTX()
+	require.Equal(t, false, mp.Verify(tx2))
+	require.Error(t, mp.Add(tx2, &FeerStub{}))
+}
+
+func newIssueTX() *transaction.Transaction {
+	return &transaction.Transaction{
+		Type: transaction.IssueType,
+		Data: &transaction.IssueTX{},
+		Outputs: []transaction.Output{
+			transaction.Output{
+				AssetID:    random.Uint256(),
+				Amount:     util.Fixed8FromInt64(42),
+				ScriptHash: random.Uint160(),
+			},
+		},
+	}
+}
+
 func newMinerTX(i uint32) *transaction.Transaction {
 	return &transaction.Transaction{
 		Type: transaction.MinerType,