PAYMENT-683 - Reducing number of open files in Archiver

Lowering the value of MAX_FILES_OPEN from 5 to 2 in order to reduce
the latency for slot files to be available on S3 (since they cannot be
uploaded until they are closed).

Ref. https://confluence.skunk-works.no/jira/browse/PAYMENT-683
Merge pull request #137 from bsvingen/payment_683

PAYMENT-683 - Reducing number of open files in Archiver