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. 
I refrained from passing it as a parameter because it would bother all users of this method who are not using future. Do you foresee any issue using global ExecutionContext?