MINOR: ssl/lua: CertCache.set() allows to update an SSL certificate file

MINOR: ssl/lua: CertCache.set() allows to update an SSL certificate file

The CertCache.set() function allows to update an SSL certificate file
stored in the memory of the HAProxy process. This function does the same
as "set ssl cert" + "commit ssl cert" over the CLI.

This could be used to update the crt and key, as well as the OCSP, the
SCTL, and the OSCP issuer.

The implementation does yield every 10 ckch instances, the same way the
"commit ssl cert" do.