TUN-8861: Add session limiter to UDP session manager

## Summary
In order to make cloudflared behavior more predictable and
prevent an exhaustion of resources, we have decided to add
session limits that can be configured by the user. This first
commit introduces the session limiter and adds it to the UDP
handling path. For now the limiter is set to run only in
unlimited mode.

vendor/golang.org/x/net/http2/http2.go

@@ -17,6 +17,7 @@ package http2 // import "golang.org/x/net/http2"
 
 import (
 	"bufio"
+	"context"
 	"crypto/tls"
 	"fmt"
 	"io"
@@ -26,6 +27,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"time"
 
 	"golang.org/x/net/http/httpguts"
 )
@@ -210,12 +212,6 @@ type stringWriter interface {
 	WriteString(s string) (n int, err error)
 }
 
-// A gate lets two goroutines coordinate their activities.
-type gate chan struct{}
-
-func (g gate) Done() { g <- struct{}{} }
-func (g gate) Wait() { <-g }
-
 // A closeWaiter is like a sync.WaitGroup but only goes 1 to 0 (open to closed).
 type closeWaiter chan struct{}
 
@@ -383,3 +379,14 @@ func validPseudoPath(v string) bool {
 // makes that struct also non-comparable, and generally doesn't add
 // any size (as long as it's first).
 type incomparable [0]func()
+
+// synctestGroupInterface is the methods of synctestGroup used by Server and Transport.
+// It's defined as an interface here to let us keep synctestGroup entirely test-only
+// and not a part of non-test builds.
+type synctestGroupInterface interface {
+	Join()
+	Now() time.Time
+	NewTimer(d time.Duration) timer
+	AfterFunc(d time.Duration, f func()) timer
+	ContextWithTimeout(ctx context.Context, d time.Duration) (context.Context, context.CancelFunc)
+}