Merge #21016: refactor: remove boost::thread_group usage

dc8be12510 refactor: remove boost::thread_group usage (fanquake)

Pull request description:

  Post #18710, there isn't much left using `boost::thread_group`, so should just be able to replace it with the standard library. This also removes the last use of `boost::thread_interrupted`.

  After this change, last piece of Boost Thread we'd be using is `boost::shared_mutex`. See the commentary [here](https://github.com/bitcoin/bitcoin/issues/16684#issuecomment-726214696) as to why it may be non-trivial to swap that for `std::shared_mutex` in the near future.

  Closes #17307

ACKs for top commit:
  laanwj:
    Code review re-ACK dc8be12510
  MarcoFalke:
    review ACK dc8be12510 🔁
  jonatack:
    Non-expert code review ACK dc8be12510, also checked range-diff since last review and that local debug build is clean with gcc 10.2.1-6 on Debian

Tree-SHA512: 5510e2d760cce824234207dc86b1551ca8f21cbf3a2ce753c0254a0d03ffd83c94e449aec202fb7bd76e6fc64df783a6b70a736b0add9ece3734bb9c8ce8fc2f

src/init.cpp

@@ -68,6 +68,8 @@
 #include <set>
 #include <stdint.h>
 #include <stdio.h>
+#include <thread>
+#include <vector>
 
 #ifndef WIN32
 #include <attributes.h>
@@ -78,7 +80,6 @@
 
 #include <boost/algorithm/string/replace.hpp>
 #include <boost/signals2/signal.hpp>
-#include <boost/thread/thread.hpp>
 
 #if ENABLE_ZMQ
 #include <zmq/zmqabstractnotifier.h>
@@ -155,8 +156,6 @@ static std::unique_ptr<ECCVerifyHandle> globalVerifyHandle;
 
 static std::thread g_load_block;
 
-static boost::thread_group threadGroup;
-
 void Interrupt(NodeContext& node)
 {
     InterruptHTTPServer();
@@ -218,11 +217,9 @@ void Shutdown(NodeContext& node)
     StopTorControl();
 
     // After everything has been shut down, but before things get flushed, stop the
-    // CScheduler/checkqueue, threadGroup and load block thread.
+    // CScheduler/checkqueue, scheduler and load block thread.
     if (node.scheduler) node.scheduler->stop();
     if (g_load_block.joinable()) g_load_block.join();
-    threadGroup.interrupt_all();
-    threadGroup.join_all();
     StopScriptCheckWorkerThreads();
 
     // After the threads that potentially access these pointers have been stopped,
@@ -1342,7 +1339,7 @@ bool AppInitMain(const util::Ref& context, NodeContext& node, interfaces::BlockA
     node.scheduler = MakeUnique<CScheduler>();
 
     // Start the lightweight task scheduler thread
-    threadGroup.create_thread([&] { TraceThread("scheduler", [&] { node.scheduler->serviceQueue(); }); });
+    node.scheduler->m_service_thread = std::thread([&] { TraceThread("scheduler", [&] { node.scheduler->serviceQueue(); }); });
 
     // Gather some entropy once per minute.
     node.scheduler->scheduleEvery([]{