use a sync.Pool to reduce allocation of linked list elements

Especially the sentPacketHistory linked list shows up in allocation
profiles, since a new list element is allocated for every single packet
we send.
Using a pool for the receiving path, as well as for the frame sorter, is
less critical, since we're tracking ranges there instead of individual
packets / frames, but it doesn't hurt either.
The other occurrences where we use a linked list (connection ID tracking
and the token store) are used so rarely (a few times over the lifetime
of the connection) that using a pool doesn't make any sense there.

internal/ackhandler/received_packet_history.go

@@ -1,6 +1,8 @@
 package ackhandler
 
 import (
+	"sync"
+
 	"github.com/lucas-clemente/quic-go/internal/protocol"
 	list "github.com/lucas-clemente/quic-go/internal/utils/linkedlist"
 	"github.com/lucas-clemente/quic-go/internal/wire"
@@ -12,6 +14,12 @@ type interval struct {
 	End   protocol.PacketNumber
 }
 
+var intervalElementPool sync.Pool
+
+func init() {
+	intervalElementPool = *list.NewPool[interval]()
+}
+
 // The receivedPacketHistory stores if a packet number has already been received.
 // It generates ACK ranges which can be used to assemble an ACK frame.
 // It does not store packet contents.
@@ -23,7 +31,7 @@ type receivedPacketHistory struct {
 
 func newReceivedPacketHistory() *receivedPacketHistory {
 	return &receivedPacketHistory{
-		ranges: list.New[interval](),
+		ranges: list.NewWithPool[interval](&intervalElementPool),
 	}
 }
 

internal/ackhandler/sent_packet_history.go

@@ -2,6 +2,7 @@ package ackhandler
 
 import (
 	"fmt"
+	"sync"
 	"time"
 
 	"github.com/lucas-clemente/quic-go/internal/protocol"
@@ -17,11 +18,17 @@ type sentPacketHistory struct {
 	highestSent           protocol.PacketNumber
 }
 
+var packetElementPool sync.Pool
+
+func init() {
+	packetElementPool = *list.NewPool[*Packet]()
+}
+
 func newSentPacketHistory(rttStats *utils.RTTStats) *sentPacketHistory {
 	return &sentPacketHistory{
 		rttStats:              rttStats,
-		outstandingPacketList: list.New[*Packet](),
-		etcPacketList:         list.New[*Packet](),
+		outstandingPacketList: list.NewWithPool[*Packet](&packetElementPool),
+		etcPacketList:         list.NewWithPool[*Packet](&packetElementPool),
 		packetMap:             make(map[protocol.PacketNumber]*list.Element[*Packet]),
 		highestSent:           protocol.InvalidPacketNumber,
 	}