mempool: replace timeStamp with blockStamp

Time is not really relevant for us here and we don't use this timestamp in any
way. Yet it occupies 24 bytes and we do two clock_gettime calls to get it.

Replace it with blockStamp which is going to be used in the future for
transaction retransmissions.

It allows to improve single-node TPS by another 3%.

pkg/core/blockchainer.go

@@ -19,7 +19,6 @@ type Blockchainer interface {
 	AddHeaders(...*block.Header) error
 	AddBlock(*block.Block) error
 	AddStateRoot(r *state.MPTRoot) error
-	BlockHeight() uint32
 	CalculateClaimable(value util.Fixed8, startHeight, endHeight uint32) (util.Fixed8, util.Fixed8, error)
 	Close()
 	HeaderHeight() uint32

pkg/core/mempool/feer.go

@@ -7,6 +7,7 @@ import (
 
 // Feer is an interface that abstract the implementation of the fee calculation.
 type Feer interface {
+	BlockHeight() uint32
 	NetworkFee(t *transaction.Transaction) util.Fixed8
 	IsLowPriority(util.Fixed8) bool
 	FeePerByte(t *transaction.Transaction) util.Fixed8

pkg/core/mempool/mem_pool.go

@@ -4,7 +4,6 @@ import (
 	"errors"
 	"sort"
 	"sync"
-	"time"
 
 	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
 	"github.com/nspcc-dev/neo-go/pkg/util"
@@ -26,7 +25,7 @@ var (
 // item represents a transaction in the the Memory pool.
 type item struct {
 	txn        *transaction.Transaction
-	timeStamp  time.Time
+	blockStamp uint32
 	perByteFee util.Fixed8
 	netFee     util.Fixed8
 	isLowPrio  bool
@@ -162,7 +161,7 @@ func dropInputFromSortedSlice(slice *[]*transaction.Input, input *transaction.In
 func (mp *Pool) Add(t *transaction.Transaction, fee Feer) error {
 	var pItem = &item{
 		txn:        t,
-		timeStamp:  time.Now().UTC(),
+		blockStamp: fee.BlockHeight(),
 		perByteFee: fee.FeePerByte(t),
 		netFee:     fee.NetworkFee(t),
 	}

pkg/core/mempool/mem_pool_test.go

@@ -18,6 +18,10 @@ type FeerStub struct {
 	perByteFee  util.Fixed8
 }
 
+func (fs *FeerStub) BlockHeight() uint32 {
+	return 0
+}
+
 func (fs *FeerStub) NetworkFee(*transaction.Transaction) util.Fixed8 {
 	return fs.netFee
 }

pkg/rpc/server/server_helper_test.go

@@ -85,6 +85,10 @@ func initServerWithInMemoryChain(t *testing.T) (*core.Blockchain, *Server, *http
 
 type FeerStub struct{}
 
+func (fs *FeerStub) BlockHeight() uint32 {
+	return 0
+}
+
 func (fs *FeerStub) NetworkFee(*transaction.Transaction) util.Fixed8 {
 	return 0
 }