Use size_t offset type in CheckpointManager::putCursorsInChk()

Change-Id: I8fc218d7baa8cc5d87fb171694bb6748f822f64a
Reviewed-on: http://review.couchbase.org/34483
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>