Prevent unload of modules which implement an Optional API.

Once an Optional API module is loaded it should stay loaded.  Unloading
an optional API module runs the risk of a crash if something else is
using it.  This patch causes all optional API providers to tell the
module loader not to unload except at shutdown.

ASTERISK-27389

Change-Id: Ia07786fe655681aec49cc8d3d96e06483b11f5e6

res/res_statsd.c

@@ -353,6 +353,9 @@ static int load_module(void)
 		return AST_MODULE_LOAD_DECLINE;
 	}
 
+	/* For Optional API. */
+	ast_module_shutdown_ref(AST_MODULE_SELF);
+
 	return AST_MODULE_LOAD_SUCCESS;
 }