Make LazyLoad.render wrap passed functions in deferred contexts.

Initially, the LazyLoad.render overload that took a (String)=>JsCmd
explicitly didn't wrap buildDeferredFunction around the function, but
after thinking about it a little more, not needing buildDeferredFunction
is low-level and rare enough that we'll leave it so using that requires
using AsyncRenderComet.asyncRenderDeferred instead.