Initial commit for conditional put on metadata.start for latest/ table

[ABPLMC]

Initial commit for upgrading the 'latest' functionality of datalake. This PR is specifically for the updates to the ingester.storage file and associated tests.

Note this PR is written for boto2. We will likely want to upgrade to boto3 which has support for a ConditionExpression parameter passed to the table.put_item method. Ultimately, the same functionality will be present.

Helpful links: https://boto.cloudhackers.com/en/latest/ref/dynamodb2.html#boto.dynamodb2.layer1.DynamoDBConnection.put_item https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_AttributeValue.html