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/nettest/conntest.go

@@ -8,7 +8,6 @@ import (
 	"bytes"
 	"encoding/binary"
 	"io"
-	"io/ioutil"
 	"math/rand"
 	"net"
 	"runtime"
@@ -173,7 +172,7 @@ func testRacyRead(t *testing.T, c1, c2 net.Conn) {
 // testRacyWrite tests that it is safe to mutate the input Write buffer
 // immediately after cancelation has occurred.
 func testRacyWrite(t *testing.T, c1, c2 net.Conn) {
-	go chunkedCopy(ioutil.Discard, c2)
+	go chunkedCopy(io.Discard, c2)
 
 	var wg sync.WaitGroup
 	defer wg.Wait()
@@ -200,7 +199,7 @@ func testRacyWrite(t *testing.T, c1, c2 net.Conn) {
 
 // testReadTimeout tests that Read timeouts do not affect Write.
 func testReadTimeout(t *testing.T, c1, c2 net.Conn) {
-	go chunkedCopy(ioutil.Discard, c2)
+	go chunkedCopy(io.Discard, c2)
 
 	c1.SetReadDeadline(aLongTimeAgo)
 	_, err := c1.Read(make([]byte, 1024))