Request new firmware URL if current is expired

[jjcarstens]

Moving this from nerves-hub/nerves_hub_web#595

TL;DR - If you get a firmware url and delay applying it before the TTL expires (~10 minutes), then issues happening.

We should adapt the download request here to ask for a new, generated URL if the link is expired, but limit to only X number of retries (3 maybe?)

This will prob require new channel message in nerves_hub_web to support getting another download URL because currently it only happens on channel join or when deployment/device changes in nerves_hub_web