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/init.cpp

@@ -1164,7 +1164,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     }, std::chrono::minutes{5});
 
     assert(!node.validation_signals);
-    node.validation_signals = std::make_unique<ValidationSignals>(scheduler);
+    node.validation_signals = std::make_unique<ValidationSignals>(std::make_unique<SerialTaskRunner>(scheduler));
     auto& validation_signals = *node.validation_signals;
 
     // Create client interfaces for wallets that are supposed to be loaded