Dev mode: Setting up temporary credentials can hang

[austencollins]

This seems to happen immediately after provisioning the AWS IAM Role for the first time. However, it seems like our retry logic is not good enough, since it will hang in this state for 8 minutes–but after simply canceling dev mode and restarting it, this takes 200ms.

[Mmarzex]

This is most likely a case of the exponential backoff/retry getting very long while waiting for the role to become available. We can probably change just the IAM client to check more frequently so it doesn't happen.