MEDIUM: ssl: Enable backend certificate hot update

MEDIUM: ssl: Enable backend certificate hot update

When trying to update a backend certificate, we should find a
server-side ckch instance thanks to which we can rebuild a new ssl
context and a new ckch instance that replace the previous ones in the
server structure. This way any new ssl session will be built out of the
new ssl context and the newly updated certificate.

This resolves a subpart of GitHub issue #427 (the certificate part)