Fix race condition when destroying the gnb stack class.

The gnb stack class could be destroyed concurrently while still running its task scheduler thread.

srsenb/hdr/stack/gnb_stack_nr.h

@@ -106,6 +106,7 @@ public:
 private:
   void run_thread() final;
   void tti_clock_impl();
+  void stop_impl();
 
   // args
   gnb_stack_args_t        args = {};
@@ -121,7 +122,7 @@ private:
   static const int                      STACK_MAIN_THREAD_PRIO = 4;
   srsran::task_scheduler                task_sched;
   srsran::task_multiqueue::queue_handle sync_task_queue, ue_task_queue, gtpu_task_queue, mac_task_queue,
-      metrics_task_queue;
+      metrics_task_queue, gnb_task_queue;
 
   // metrics waiting condition
   std::mutex              metrics_mutex;

srsenb/src/stack/gnb_stack_nr.cc

@@ -35,6 +35,7 @@ gnb_stack_nr::gnb_stack_nr(srslog::sink& log_sink) :
   gtpu_task_queue    = task_sched.make_task_queue();
   mac_task_queue     = task_sched.make_task_queue();
   metrics_task_queue = task_sched.make_task_queue();
+  gnb_task_queue     = task_sched.make_task_queue();
 }
 
 gnb_stack_nr::~gnb_stack_nr()
@@ -97,13 +98,21 @@ int gnb_stack_nr::init(const gnb_stack_args_t& args_,
 void gnb_stack_nr::stop()
 {
   if (running) {
+    gnb_task_queue.push([this]() { stop_impl(); });
+    wait_thread_finish();
+  }
+}
+
+void gnb_stack_nr::stop_impl()
+{
   rrc.stop();
   pdcp.stop();
   mac.stop();
 
+  task_sched.stop();
   srsran::get_background_workers().stop();
+
   running = false;
-  }
 }
 
 bool gnb_stack_nr::switch_on()