[Spike] Prevent multiple concurrent checkout jobs

[xbrianh]

This is an experimental solution to the following problem: If multiple clients initiate GET requests against the same file, a checkout job will be initiated for each client.

This mechanism:

1. Places a marker in a DynamoDB table when a file checkout job is started
2. On subsequent requests: - If object exists in checkout bucket, return as usual - If not, check for marker. If marker exists, do not start new checkout job
3. Marker is deleted upon checkout success.
4. If checkout fails, marker contains the exception.