Correctly stop "own thread" in service destructor.

This change fixes the scheduler and win_iocp_io_context destructors so
that they correctly stop the internal thread that was created in the
constructor. This fixes a deadlock that can occur when two threads
concurrently attempt to create the first I/O object associated with a
non-native I/O execution context.

asio/include/asio/detail/impl/scheduler.ipp

@@ -135,6 +135,10 @@ scheduler::~scheduler()
 {
   if (thread_)
   {
+    mutex::scoped_lock lock(mutex_);
+    shutdown_ = true;
+    stop_all_threads(lock);
+    lock.unlock();
     thread_->join();
     delete thread_;
   }

asio/include/asio/detail/impl/win_iocp_io_context.ipp

@@ -113,6 +113,7 @@ win_iocp_io_context::~win_iocp_io_context()
 {
   if (thread_.get())
   {
+    stop();
     thread_->join();
     thread_.reset();
   }