swarm: verify all certificates changes

In some cases, a user in the swarm can update its certificates.
However, the new certificate MUST be checked and MUST be signed
by the account.
So, this patch validates two scenarios:
+ Check that a fetch error is sent to the client whenever an invalid
certificate is detected in the original clone
+ Check that a fetch error is sent to the client whenever a
certificate is replaced during the conversation.

Change-Id: Ieb15fb6444dcf4541f00c511a9f4ba0c64617130