pex: improve goroutine lifecycle (#7343)

I saw a race detected in a test here that I think would be better
handled by just wiring up these threads.

internal/p2p/pex/reactor.go

@@ -141,8 +141,8 @@ func NewReactor(
 // messages on that p2p channel accordingly. The caller must be sure to execute
 // OnStop to ensure the outbound p2p Channels are closed.
 func (r *Reactor) OnStart(ctx context.Context) error {
-	go r.processPexCh()
-	go r.processPeerUpdates()
+	go r.processPexCh(ctx)
+	go r.processPeerUpdates(ctx)
 	return nil
 }
 
@@ -162,17 +162,22 @@ func (r *Reactor) OnStop() {
 
 // processPexCh implements a blocking event loop where we listen for p2p
 // Envelope messages from the pexCh.
-func (r *Reactor) processPexCh() {
+func (r *Reactor) processPexCh(ctx context.Context) {
 	defer r.pexCh.Close()
-
+	timer := time.NewTimer(0)
+	defer timer.Stop()
 	for {
+		timer.Reset(time.Until(r.nextRequestTime))
+
 		select {
+		case <-ctx.Done():
+			return
 		case <-r.closeCh:
 			r.Logger.Debug("stopped listening on PEX channel; closing...")
 			return
 
 		// outbound requests for new peers
-		case <-r.waitUntilNextRequest():
+		case <-timer.C:
 			r.sendRequestForPeers()
 
 		// inbound requests for new peers or responses to requests sent by this
@@ -192,11 +197,13 @@ func (r *Reactor) processPexCh() {
 // processPeerUpdates initiates a blocking process where we listen for and handle
 // PeerUpdate messages. When the reactor is stopped, we will catch the signal and
 // close the p2p PeerUpdatesCh gracefully.
-func (r *Reactor) processPeerUpdates() {
+func (r *Reactor) processPeerUpdates(ctx context.Context) {
 	defer r.peerUpdates.Close()
 
 	for {
 		select {
+		case <-ctx.Done():
+			return
 		case peerUpdate := <-r.peerUpdates.Updates():
 			r.processPeerUpdate(peerUpdate)
 
@@ -317,10 +324,6 @@ func (r *Reactor) processPeerUpdate(peerUpdate p2p.PeerUpdate) {
 	}
 }
 
-func (r *Reactor) waitUntilNextRequest() <-chan time.Time {
-	return time.After(time.Until(r.nextRequestTime))
-}
-
 // sendRequestForPeers pops the first peerID off the list and sends the
 // peer a request for more peer addresses. The function then moves the
 // peer into the requestsSent bucket and calculates when the next request