Add automatic expiry to in-flight tiles

[rmarianski]

Sqs drops messages after some time. When this occurs, nothing removes the corresponding tile from the in-flight set. This is problematic because once we get into this state we would never re-enqueue that tile.

If we modeled the in-flight set as individual keys, we could set an expires on them to match the sqs ttl, or be slightly more conservative.

[nvkelso]

Doesn't affect the API, moving to v1.1

[nvkelso]

From a root cause analysis this week (dev and prod):

We could also modify update procedure check the queue lengths (visible, delayed, notvisible) and if they're all zero then to wipe the in-flight list.