kernel: Remove dependency on CScheduler

By defining a virtual interface class for the scheduler client, users of
the kernel can now define their own event consuming infrastructure,
without having to spawn threads or rely on the scheduler design.

Removing CScheduler also allows removing the thread and
exception modules from the kernel library.

src/bitcoin-chainstate.cpp

@@ -23,11 +23,10 @@
 #include <node/caches.h>
 #include <node/chainstate.h>
 #include <random.h>
-#include <scheduler.h>
 #include <script/sigcache.h>
 #include <util/chaintype.h>
 #include <util/fs.h>
-#include <util/thread.h>
+#include <util/task_runner.h>
 #include <validation.h>
 #include <validationinterface.h>
 
@@ -68,16 +67,7 @@ int main(int argc, char* argv[])
     Assert(InitSignatureCache(validation_cache_sizes.signature_cache_bytes));
     Assert(InitScriptExecutionCache(validation_cache_sizes.script_execution_cache_bytes));
 
-
-    // SETUP: Scheduling and Background Signals
-    CScheduler scheduler{};
-    // Start the lightweight task scheduler thread
-    scheduler.m_service_thread = std::thread(util::TraceThread, "scheduler", [&] { scheduler.serviceQueue(); });
-
-    ValidationSignals validation_signals{scheduler};
-
-    // Gather some entropy once per minute.
-    scheduler.scheduleEvery(RandAddPeriodic, std::chrono::minutes{1});
+    ValidationSignals validation_signals{std::make_unique<util::ImmediateTaskRunner>()};
 
     class KernelNotifications : public kernel::Notifications
     {
@@ -288,7 +278,6 @@ int main(int argc, char* argv[])
 epilogue:
     // Without this precise shutdown sequence, there will be a lot of nullptr
     // dereferencing and UB.
-    scheduler.stop();
     if (chainman.m_thread_load.joinable()) chainman.m_thread_load.join();
 
     validation_signals.FlushBackgroundCallbacks();