make controller_run_result become movable.

Fixes #106

This allows a consuming application to store this for later processing (i.e. for shutting down the server without terminating the application itself).
Also ensure that `controller_run_result` is not copyable.

lib/malloy/core/detail/controller_run_result.hpp

@@ -37,6 +37,7 @@ namespace malloy::detail
     };
 
     template<typename T>
+        requires std::movable<T>
     class controller_run_result
     {
     public:
@@ -65,6 +66,19 @@ namespace malloy::detail
             cfg.logger->debug("starting i/o context.");
         }
 
+        /**
+         * Move constructor.
+         */
+        controller_run_result(controller_run_result&&) noexcept = default;
+
+        /**
+         * Move-assignment operator.
+         */
+        controller_run_result& operator=(controller_run_result&&) noexcept = default;
+
+        controller_run_result(const controller_run_result&) = delete;
+        controller_run_result& operator=(const controller_run_result&) = delete;
+
         /**
          * Destructor.
          */
@@ -73,11 +87,13 @@ namespace malloy::detail
             // Stop the `io_context`. This will cause `run()`
             // to return immediately, eventually destroying the
             // `io_context` and all of the sockets in it.
-            m_io_ctx->stop();
+            if (m_io_ctx)
+                m_io_ctx->stop();
 
             // Tell the workguard that we no longer need it's service
             m_workguard.reset();
 
+            // Join threads
             for (auto& thread : m_io_threads)
                 thread.join();
         }