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/blockchainer.go

@@ -22,7 +22,6 @@ type Blockchainer interface {
 	AddHeaders(...*block.Header) error
 	AddBlock(*block.Block) error
 	AddStateRoot(r *state.MPTRoot) error
-	BlockHeight() uint32
 	CalculateClaimable(value *big.Int, startHeight, endHeight uint32) *big.Int
 	Close()
 	HeaderHeight() uint32

pkg/core/mempool/feer.go

@@ -10,4 +10,5 @@ import (
 type Feer interface {
 	FeePerByte() int64
 	GetUtilityTokenBalance(util.Uint160) *big.Int
+	BlockHeight() uint32
 }

pkg/core/mempool/mem_pool.go

@@ -5,7 +5,6 @@ import (
 	"math/big"
 	"sort"
 	"sync"
-	"time"
 
 	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
 	"github.com/nspcc-dev/neo-go/pkg/util"
@@ -31,7 +30,7 @@ var (
 // item represents a transaction in the the Memory pool.
 type item struct {
 	txn        *transaction.Transaction
-	timeStamp time.Time
+	blockStamp uint32
 }
 
 // items is a slice of item.
@@ -149,7 +148,7 @@ func checkBalance(tx *transaction.Transaction, balance utilityBalanceAndFees) er
 func (mp *Pool) Add(t *transaction.Transaction, fee Feer) error {
 	var pItem = item{
 		txn:        t,
-		timeStamp: time.Now().UTC(),
+		blockStamp: fee.BlockHeight(),
 	}
 	mp.lock.Lock()
 	if mp.containsKey(t.Hash()) {

pkg/core/mempool/mem_pool_test.go

@@ -23,6 +23,10 @@ func (fs *FeerStub) FeePerByte() int64 {
 	return fs.feePerByte
 }
 
+func (fs *FeerStub) BlockHeight() uint32 {
+	return 0
+}
+
 func (fs *FeerStub) GetUtilityTokenBalance(uint160 util.Uint160) *big.Int {
 	return balance
 }

pkg/rpc/server/server_helper_test.go

@@ -91,6 +91,10 @@ func (fs *FeerStub) FeePerByte() int64 {
 	return 0
 }
 
+func (fs *FeerStub) BlockHeight() uint32 {
+	return 0
+}
+
 func (fs *FeerStub) GetUtilityTokenBalance(acc util.Uint160) *big.Int {
 	return big.NewInt(1000000 * native.GASFactor)
 }