introduce and use the generalized node::Warnings interface

Instead of having separate warning functions (and globals) for each
different warning that can be raised, encapsulate this logic into
a single class and allow to (un)set any number of warnings.

Introduces behaviour change:
- the `-alertnotify` command is executed for all
  `KernelNotifications::warningSet` calls, which now also covers the
  `LARGE_WORK_INVALID_CHAIN` warning.
- previously, warnings were returned based on a predetermined order,
  e.g. with the "pre-release test build" warning always first. This
  is no longer the case, and Warnings::GetMessages() will return
  messages sorted by the id of the warning.

Removes warnings.cpp from kernel.

src/bitcoin-chainstate.cpp

@@ -16,6 +16,7 @@
 #include <kernel/checks.h>
 #include <kernel/context.h>
 #include <kernel/validation_cache_sizes.h>
+#include <kernel/warning.h>
 
 #include <consensus/validation.h>
 #include <core_io.h>
@@ -28,6 +29,7 @@
 #include <util/fs.h>
 #include <util/signalinterrupt.h>
 #include <util/task_runner.h>
+#include <util/translation.h>
 #include <validation.h>
 #include <validationinterface.h>
 
@@ -86,9 +88,13 @@ int main(int argc, char* argv[])
         {
             std::cout << "Progress: " << title.original << ", " << progress_percent << ", " << resume_possible << std::endl;
         }
-        void warning(const bilingual_str& warning) override
+        void warningSet(kernel::Warning id, const bilingual_str& message) override
         {
-            std::cout << "Warning: " << warning.original << std::endl;
+            std::cout << "Warning " << static_cast<int>(id) << " set: " << message.original << std::endl;
+        }
+        void warningUnset(kernel::Warning id) override
+        {
+            std::cout << "Warning " << static_cast<int>(id) << " unset" << std::endl;
         }
         void flushError(const bilingual_str& message) override
         {