refactor: De-globalize last notified header index

In future, users of the kernel library might run multiple chainstates in
parallel, or create and destroy multiple chainstates over the lifetime
of a process. Having static, mutable variables could lead to state
inconsistencies in these scenarios.

Author: TheCharlatan

src/validation.cpp

@@ -3409,16 +3409,15 @@ static bool NotifyHeaderTip(ChainstateManager& chainman) LOCKS_EXCLUDED(cs_main)
 {
     bool fNotify = false;
     bool fInitialBlockDownload = false;
-    static CBlockIndex* pindexHeaderOld = nullptr;
     CBlockIndex* pindexHeader = nullptr;
     {
         LOCK(cs_main);
         pindexHeader = chainman.m_best_header;
 
-        if (pindexHeader != pindexHeaderOld) {
+        if (pindexHeader != chainman.m_last_notified_header) {
             fNotify = true;
             fInitialBlockDownload = chainman.IsInitialBlockDownload();
-            pindexHeaderOld = pindexHeader;
+            chainman.m_last_notified_header = pindexHeader;
         }
     }
     // Send block tip changed notifications without cs_main

src/validation.h

@@ -1063,6 +1063,9 @@ class ChainstateManager
     /** Best header we've seen so far (used for getheaders queries' starting points). */
     CBlockIndex* m_best_header GUARDED_BY(::cs_main){nullptr};
 
+    /** The last header for which a headerTip notification was issued. */
+    CBlockIndex* m_last_notified_header GUARDED_BY(::cs_main){nullptr};
+
     //! The total number of bytes available for us to use across all in-memory
     //! coins caches. This will be split somehow across chainstates.
     int64_t m_total_coinstip_cache{0};