make reload_handles optional for creating Services

This will be useful for instantiating services in CLI subcommands, which
have different requirements around observeability.

src/service.rs

@@ -62,7 +62,7 @@ impl Services {
     >(
         db: &'static D,
         config: Config,
-        reload_handles: FilterReloadHandles,
+        reload_handles: Option<FilterReloadHandles>,
     ) -> Result<Self> {
         Ok(Self {
             appservice: appservice::Service::build(db)?,