Notify users about auto-senescence via -alertnotify

Closes #2829.

src/deprecation.h

@@ -18,8 +18,11 @@ static const int DEPRECATION_WARN_LIMIT = 14 * 24 * 24; // 2 weeks
 /**
  * Checks whether the node is deprecated based on the current block height, and
  * shuts down the node with an error if so (and deprecation is not disabled for
- * the current client version).
+ * the current client version). Warning and error messages are sent to the debug
+ * log, the metrics UI, and (if configured) -alertnofity.
+ *
+ * fThread means run -alertnotify in a free-running thread.
  */
-void EnforceNodeDeprecation(int nHeight, bool forceLogging=false);
+void EnforceNodeDeprecation(int nHeight, bool forceLogging=false, bool fThread=true);
 
 #endif // ZCASH_DEPRECATION_H