Explicitly stop IoContext in Controller

By stopping the Controllers IoContext in the destructor we make sure that no
scheduled task tries to access a deleted resource from DiscoveryService while
it is being deleted.

include/ableton/link/Controller.hpp

@@ -169,6 +169,11 @@ class Controller
   Controller& operator=(const Controller&) = delete;
   Controller& operator=(Controller&&) = delete;
 
+  ~Controller()
+  {
+    mIo->stop();
+  }
+
   void enable(const bool bEnable)
   {
     const bool bWasEnabled = mEnabled.exchange(bEnable);

include/ableton/platforms/asio/Context.hpp

@@ -87,13 +87,24 @@ class Context
 
   ~Context()
   {
-    if (mpService)
+    if (mpService && mpWork)
     {
       mpWork.reset();
       mThread.join();
     }
   }
 
+  void stop()
+  {
+    if (mpService && mpWork)
+    {
+      mpWork.reset();
+      mpService->stop();
+      mThread.join();
+    }
+  }
+
+
   template <std::size_t BufferSize>
   Socket<BufferSize> openUnicastSocket(const ::asio::ip::address_v4& addr)
   {

include/ableton/platforms/esp32/Context.hpp

@@ -114,6 +114,10 @@ class Context
   {
   }
 
+  void stop()
+  {
+  }
+
   template <std::size_t BufferSize>
   Socket<BufferSize> openUnicastSocket(const ::asio::ip::address_v4& addr)
   {

include/ableton/test/serial_io/Context.hpp

@@ -67,6 +67,10 @@ class Context
   {
   }
 
+  void stop()
+  {
+  }
+
   template <typename Handler>
   void async(Handler handler)
   {

src/ableton/link/tst_Controller.cpp

@@ -84,6 +84,10 @@ struct MockIoContext
     }
   };
 
+  void stop()
+  {
+  }
+
   template <std::size_t BufferSize>
   Socket<BufferSize> openUnicastSocket(const asio::ip::address_v4&)
   {