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/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)
 }