CCBC-543: Use settings->durability_interval if not specified

Don't use the hardcoded default

Change-Id: I223f51d8c1a0a6099c172ef84997e8642097cadc
Reviewed-on: http://review.couchbase.org/42906
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>