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/validation.h

@@ -42,7 +42,6 @@
 #include <span>
 #include <stdint.h>
 #include <string>
-#include <thread>
 #include <type_traits>
 #include <utility>
 #include <vector>
@@ -1008,7 +1007,6 @@ public:
 
     const util::SignalInterrupt& m_interrupt;
     const Options m_options;
-    std::thread m_thread_load;
     //! A single BlockManager instance is shared across each constructed
     //! chainstate to avoid duplicating block metadata.
     node::BlockManager m_blockman;