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/validation.cpp

@@ -27,11 +27,11 @@
 #include <kernel/mempool_entry.h>
 #include <kernel/messagestartchars.h>
 #include <kernel/notifications_interface.h>
+#include <kernel/warning.h>
 #include <logging.h>
 #include <logging/timer.h>
 #include <node/blockstorage.h>
 #include <node/utxo_snapshot.h>
-#include <node/warnings.h>
 #include <policy/policy.h>
 #include <policy/rbf.h>
 #include <policy/settings.h>
@@ -1922,9 +1922,11 @@ void Chainstate::CheckForkWarningConditions()
 
     if (m_chainman.m_best_invalid && m_chainman.m_best_invalid->nChainWork > m_chain.Tip()->nChainWork + (GetBlockProof(*m_chain.Tip()) * 6)) {
         LogPrintf("%s: Warning: Found invalid chain at least ~6 blocks longer than our best chain.\nChain state database corruption likely.\n", __func__);
-        node::SetfLargeWorkInvalidChainFound(true);
+        m_chainman.GetNotifications().warningSet(
+            kernel::Warning::LARGE_WORK_INVALID_CHAIN,
+            _("Warning: We do not appear to fully agree with our peers! You may need to upgrade, or other nodes may need to upgrade."));
     } else {
-        node::SetfLargeWorkInvalidChainFound(false);
+        m_chainman.GetNotifications().warningUnset(kernel::Warning::LARGE_WORK_INVALID_CHAIN);
     }
 }
 
@@ -2907,7 +2909,7 @@ void Chainstate::UpdateTip(const CBlockIndex* pindexNew)
             if (state == ThresholdState::ACTIVE || state == ThresholdState::LOCKED_IN) {
                 const bilingual_str warning = strprintf(_("Unknown new rules activated (versionbit %i)"), bit);
                 if (state == ThresholdState::ACTIVE) {
-                    m_chainman.GetNotifications().warning(warning);
+                    m_chainman.GetNotifications().warningSet(kernel::Warning::UNKNOWN_NEW_RULES_ACTIVATED, warning);
                 } else {
                     warning_messages.push_back(warning);
                 }