Add TTL to DynamoDB so that stale locks expire (albeit silently, for data cleanup purposes)

[connorads]

Locks last forever and will never be cleaned up. Even random locks in users DMs or random dead channels. Might be good to get rid of these?

Two weeks?
https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/TTL.html

[connorads]

Maybe this would be covered by #7 with a default expiry value.