kernel: Move background load thread to node context

The thread handle is never used by the ChainstateManager, so move it out
and into the node context. Users of the kernel library now no longer
have to manually join the thread when destructing the ChainstateManager.

src/node/context.h

@@ -9,6 +9,7 @@
 #include <cstdlib>
 #include <functional>
 #include <memory>
+#include <thread>
 #include <vector>
 
 class ArgsManager;
@@ -86,6 +87,7 @@ struct NodeContext {
     std::atomic<int> exit_status{EXIT_SUCCESS};
     //! Manages all the node warnings
     std::unique_ptr<node::Warnings> warnings;
+    std::thread background_init_thread;
 
     //! Declare default constructor and destructor that are not inline, so code
     //! instantiating the NodeContext struct doesn't need to #include class