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`?
Uh, do you not care on whether you got a satisfactory response before retrying? This sounds like you'll be leaking resources you acquire-then-ignore?