Implement delayed cleanup support

[zanerock]

Overview

When stacks are destroyed, you can't always delete everything right away. So, when the delete fails, we want to go ahead and delete the sites (assuming #92 is done first) entry because the site is no longer valid. We transfer the site data to a toCleanup array

We would then implement a 'cleanup' command that will iterate through toCleanup sites (or, delete the specified site) and try to delete them. If unsuccessful, add/update a lastCleanupAttempt for the item entry.

Deliverables

• Implement cleanup (all)
• Implement cleanup XXX
• Implement cleanup --list which will list the sites, if any
• make on entry in reminders and remove entry once site is deleted

[zanerock]

Work for this issue will begin begin on branch work-liquid-labs/cloudsite/93.