iops: use float conversion when converting usec to seconds

This otherwise messes up timeouts when the actual interval is < 0, in
which case it ends up being 0 and firing immediately.

Change-Id: I75f34568212779a5711bb4addc495c25ea520ec4
Reviewed-on: http://review.couchbase.org/43580
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>