certstore: make getCertificate always construct the complete cert

Because the certstore can contains a certificate chain splitted
in different parts (eg, in a swarm the device cert is in /devices
and the rest in /members), the certstore should be able to return
the whole certificate.
Moreover, at some places we do the assumption that the cert is
complete (AccountManager::foundPeerDevice which uses cert->issuer)
getCertificate() should only return the certificate if complete.

Change-Id: I1ff2006cb4f63694016e38e5b4a72b6a287c2345
GitLab: https://git.jami.net/savoirfairelinux/ring-project/-/issues/1282