Accounts can see their certificates expire. When it's the case, a
migration is needed. However, several regressions can happen, because
this behaviour can be tricky to test correctly. In this patch, a
test is added to validate that the migration is done whenever the
certificate expires.

+ OpenDHT needs to be bumped in order to be able to change the validity
period of a certificate.
+ In ArchiveAccountManager, a method is added to change the validity
of a certificate in the chain.
+ This patch also fixes a crash when a migration occurs directly on
the archive (info_ was null causing a segfault).
+ Finally, cleanup some signatures unused in JamiAccount.

GitLab: #684 (this is some preparative work)
Change-Id: I901bc67fd63ce2ab26ded64662f8333d3a0eed50