Support more than one CScheduler thread for serial clients

This will be used by CValidationInterface soon.

This requires a bit of work as we need to ensure that most of our
callbacks happen in-order (to avoid synchronization issues in
wallet) - we keep our own internal queue and push things onto it,
scheduling a queue-draining function immediately upon new
callbacks.

src/validationinterface.cpp

@@ -6,6 +6,11 @@
 #include "validationinterface.h"
 #include "init.h"
 #include "scheduler.h"
+#include "sync.h"
+#include "util.h"
+
+#include <list>
+#include <atomic>
 
 #include <boost/signals2/signal.hpp>
 
@@ -20,22 +25,23 @@ struct MainSignalsInstance {
     boost::signals2::signal<void (const CBlock&, const CValidationState&)> BlockChecked;
     boost::signals2::signal<void (const CBlockIndex *, const std::shared_ptr<const CBlock>&)> NewPoWValidBlock;
 
-    CScheduler *m_scheduler = NULL;
+    // We are not allowed to assume the scheduler only runs in one thread,
+    // but must ensure all callbacks happen in-order, so we end up creating
+    // our own queue here :(
+    SingleThreadedSchedulerClient m_schedulerClient;
+
+    MainSignalsInstance(CScheduler *pscheduler) : m_schedulerClient(pscheduler) {}
 };
 
 static CMainSignals g_signals;
 
-CMainSignals::CMainSignals() {
-    m_internals.reset(new MainSignalsInstance());
-}
-
 void CMainSignals::RegisterBackgroundSignalScheduler(CScheduler& scheduler) {
-    assert(!m_internals->m_scheduler);
-    m_internals->m_scheduler = &scheduler;
+    assert(!m_internals);
+    m_internals.reset(new MainSignalsInstance(&scheduler));
 }
 
 void CMainSignals::UnregisterBackgroundSignalScheduler() {
-    m_internals->m_scheduler = NULL;
+    m_internals.reset(nullptr);
 }
 
 CMainSignals& GetMainSignals()