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.

orchestration/orchestrator.go

@@ -10,6 +10,8 @@ import (
 	"github.com/pkg/errors"
 	"github.com/rs/zerolog"
 
+	cfdsession "github.com/cloudflare/cloudflared/session"
+
 	"github.com/cloudflare/cloudflared/config"
 	"github.com/cloudflare/cloudflared/connection"
 	"github.com/cloudflare/cloudflared/ingress"
@@ -33,7 +35,9 @@ type Orchestrator struct {
 	// cloudflared Configuration
 	config *Config
 	tags   []pogs.Tag
-	log    *zerolog.Logger
+	// sessionLimiter tracks active sessions across the tunnel and limits new sessions if they are above the limit.
+	sessionLimiter cfdsession.Limiter
+	log            *zerolog.Logger
 
 	// orchestrator must not handle any more updates after shutdownC is closed
 	shutdownC <-chan struct{}
@@ -54,6 +58,7 @@ func NewOrchestrator(ctx context.Context,
 		internalRules:  internalRules,
 		config:         config,
 		tags:           tags,
+		sessionLimiter: cfdsession.NewLimiter(0),
 		log:            log,
 		shutdownC:      ctx.Done(),
 	}
@@ -208,6 +213,12 @@ func (o *Orchestrator) GetOriginProxy() (connection.OriginProxy, error) {
 	return proxy, nil
 }
 
+// GetSessionLimiter returns the session limiter used across cloudflared, that can be hot reload when
+// the configuration changes.
+func (o *Orchestrator) GetSessionLimiter() cfdsession.Limiter {
+	return o.sessionLimiter
+}
+
 func (o *Orchestrator) waitToCloseLastProxy() {
 	<-o.shutdownC
 	o.lock.Lock()