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/bitcoin-chainstate.cpp

@@ -283,8 +283,6 @@ int main(int argc, char* argv[])
 epilogue:
     // Without this precise shutdown sequence, there will be a lot of nullptr
     // dereferencing and UB.
-    if (chainman.m_thread_load.joinable()) chainman.m_thread_load.join();
-
     validation_signals.FlushBackgroundCallbacks();
     {
         LOCK(cs_main);