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. 
Adding a comment here was confusing. I meant code here is reasonable but we cannot deprecate `futureUsing` because other `ExecutionContext` values (e.g. Akka's one) should be used when library users work with `Future`.