In a previous commit this test was disabled because I though the
feature was broken, but in fact this is the test which is broken.
Indeed the connection between the server and the client was not
renegociated and was using the SSL cache or a ticket. To be work
correctly these 2 features must be disabled or a new connection must be
established after the ticket timeout, which is too long for a regtest.
Also a "nbthread 1" was added as it was easier to reproduce the problem
with it.
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)
