Add 'replacedBy' field in certificate response.

[kevgliss]

In addition to the 'replaces' field, we should also have a field of 'replacedBy' so we can easily determine if a certificate has been replaced and should no longer be used.

[jeremyguarini]

yes, having a field that links/points to the most recent version of the cert would be very helpful. Say if cert 2 is replaced by cert 4 with is then renewed/reissued by cert 55. The replaced by field in cert 2 should either point to cert 55 or show that it was replaced by cert 4 then 55. ie. Replaced By: 4 -> 55

This would help to show the history of the cert along with what the current version is. I see that in the API output cert 55 contains the certs it replaces which is handy.

[kevgliss]

Field has been named replaced.