Add option to retry resource checkout

@ferd Hey, we are experiencing a lot of checkout misses, even though there are resources available. So we were thinking about a possibility to increase our chances for checkout a resource by retrying.
I added an option `checkout_retry`. What do you think about it? Is it viable for `dlhttpc`?
Actually, what about:

```erlang
checkout(Info, 0) ->
    dispcount:checkout(Info);
checkout(Info, Retries) ->
    case dispcount:checkout(Info) of
        {error, _} -> checkout(Info, Retries-1);
        Response -> Response
    end.
```

You can then go back and change

```erlang
CheckoutRetry = 1 + proplists:get_value(checkout_retry, Options, 0),
```
to
```erlang
CheckoutRetry = proplists:get_value(checkout_retry, Options, 0),
```
Which means the `checkout_retry` option is really about retries, and not a misnomer for 'attempts', and we avoid creating closures in a loop as a way to work-around that distinction.