Use lockfile rather than deleting candidate manifest

Change-Id: I987b97bbb58aa3f9f2f00a0f0db0ed6948e62ddf
Reviewed-on: http://review.couchbase.org/34370
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>