moduleapi: Add module:once() to execute a function after module load/startup

It is a common pattern for modules to do something like check for
prosody.start_time, and execute code immediately if it is present, or wait for
the server-started event if it isn't yet. For example, this allows you to run
code after all other modules/hosts have been loaded, that are going to be
loaded.

Such code can now be replaced with a simple call to this method.

core/moduleapi.lua

@@ -693,4 +693,9 @@ function api:may(action, context, peek)
 	end
 end
 
+-- Execute a function, once, but only after startup is complete
+function api:once(f)
+	return prosody.started:next(f);
+end
+
 return api;