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/kernel/notifications_interface.h

@@ -16,6 +16,8 @@ namespace kernel {
 
 //! Result type for use with std::variant to indicate that an operation should be interrupted.
 struct Interrupted{};
+enum class Warning;
+
 
 //! Simple result type for functions that need to propagate an interrupt status and don't have other return values.
 using InterruptResult = std::variant<std::monostate, Interrupted>;
@@ -38,7 +40,8 @@ public:
     [[nodiscard]] virtual InterruptResult blockTip(SynchronizationState state, CBlockIndex& index) { return {}; }
     virtual void headerTip(SynchronizationState state, int64_t height, int64_t timestamp, bool presync) {}
     virtual void progress(const bilingual_str& title, int progress_percent, bool resume_possible) {}
-    virtual void warning(const bilingual_str& warning) {}
+    virtual void warningSet(Warning id, const bilingual_str& message) {}
+    virtual void warningUnset(Warning id) {}
 
     //! The flush error notification is sent to notify the user that an error
     //! occurred while flushing block data to disk. Kernel code may ignore flush