fix `AsyncGossiper` hangup (#10654)

* fix AsyncGossiper hangup

* Use atomic operation when starting.

---------
Co-authored-by: Adrian Sutton <adrian@oplabs.co>

op-node/rollup/async/asyncgossiper.go

@@ -90,6 +90,11 @@ func (p *SimpleAsyncGossiper) Clear() {
 // Stop is a synchronous function to stop the async routine
 // it blocks until the async routine accepts the signal
 func (p *SimpleAsyncGossiper) Stop() {
+	// if the gossiping isn't running, nothing to do
+	if !p.running.Load() {
+		return
+	}
+
 	p.stop <- struct{}{}
 }
 
@@ -97,10 +102,9 @@ func (p *SimpleAsyncGossiper) Stop() {
 // each behavior of the loop is handled by a select case on a channel, plus an internal handler function call
 func (p *SimpleAsyncGossiper) Start() {
 	// if the gossiping is already running, return
-	if p.running.Load() {
+	if !p.running.CompareAndSwap(false, true) {
 		return
 	}
-	p.running.Store(true)
 	// else, start the handling loop
 	go func() {
 		defer p.running.Store(false)