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.

src/validation.h

@@ -1063,6 +1063,9 @@ public:
     /** 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};