CCBC-527: Durability timer fixes

Change-Id: I1d0a9138f1018b1408fc8e89b452fbbbcbe0dbc5
Reviewed-on: http://review.couchbase.org/42284
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>