Actually block when syncing subscriptions (#13066)

By changing to use WatchCtx we will actually block for changes to the peering list. WatchCh creates a goroutine to collect errors from WatchCtx and returns immediately.

The existing behavior wouldn't result in a tight loop because of the rate limiting in the surrounding function, but it would still lead to more work than is necessary.

agent/rpc/peering/subscription_manager.go

@@ -134,7 +134,7 @@ func (m *subscriptionManager) syncSubscriptionsAndBlock(ctx context.Context, pee
 	}
 
 	// Block for any changes to the state store.
-	ws.WatchCh(ctx)
+	ws.WatchCtx(ctx)
 	return nil
 }