Implement Transactionality in DynamoDB

[joshprzybyszewski]

With the addition of DynamoDB in #101, we did not implement transactionality. we left a few key areas to address:

cribbage/server/persistence/persistence_test.go

Line 659 in 978c0c6

/* TODO we'll address transactionality of dynamodb in a followup */

cribbage/server/persistence/dynamo/dynamodb.go

Lines 104 to 122 in 978c0c6

	func (dw *dynamoWrapper) Close() error {
	// TODO I don't think there's anything to do?
	return nil
	}
	func (dw *dynamoWrapper) Start() error {
	// TODO figure out transactionality in dynamodb
	return nil
	}
	func (dw *dynamoWrapper) Commit() error {
	// TODO figure out transactionality in dynamodb
	return nil
	}
	func (dw *dynamoWrapper) Rollback() error {
	// TODO figure out transactionality in dynamodb
	return nil
	}