All URLHandler usage goes through getDefault.

* All URLHandler access goes through the same method
* Removes the reflective lookup of httpclient.

Review by @eed3si9n 
If you're picking ONE name, `getHttp` is a better name than `getDefault` since the term "default" indicates that there's some sort of non-default. Un-deprecate `getHttp`, unify all calls to `getHttp`, and then we can come back to changing the imple of that to sync or async later.