INV-393 - Restrict size of batch query result from Dynamo

@clicktravel-basharat As per Matthew we need max 500 results at a time. This is a stop gap fix to make sure that places where we have too many results coming back do not crash cause of the payload size.
Please review, thanks.
Signed-off-by: Tajinder Singh <tajinder.singh@clicktravel.com>
this magic number is used in both the code and the test, should it be broken out into a static value?