LoanPattern.using should be able to deal with Futures

The following snippet :
`DB.readonly {session => Future{...}}`
fails because `LoanPattern.using` will close any resources before returning.

This behavior is unexpected and this MR aims to fix it. 
How about adding `(implicit ec: ExecutionContext = Global)` to `#using`? That way it wouldn't bother people who aren't using futures.