plugin: rescan restarts plugin on update

This adds a `u32 checksum` field to the plugin struct that is used
to identify if a plugin is outdated and needs to be restarted on `rescan`.

Note: Only affects non-important plugins.

Changelog-Added: Plugin: Restart plugin on `rescan` when binary was changed.

lightningd/plugin.h

@@ -48,6 +48,7 @@ struct plugin {
 
 	pid_t pid;
 	char *cmd;
+	u32 checksum;
 	struct io_conn *stdin_conn, *stdout_conn;
 	struct plugins *plugins;
 	const char **plugin_path;