Optional userinfo ttl

[richhaase]

Issue #, if available:

Description of changes:

This PR adds the ability to set a TTL on records added to the DynamoDB UsersTable by adding:

• a TimeToLiveSpecification to the UserTable setting the TTL field to ttl
• a setting called USERINFO_TTL_DAYS
• functionality to the fulfillment lambda which causes it to set an appropriate ttl on user cache records every time a new interaction occurs, IF the USERINFO_TTL_DAYS setting is greater than zero

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[marcburnie]

Thanks @richhaase for contributing! This looks like a useful feature. We will review and get back to you.