Flush the SSL session cache when updating a certificate which is used on a server line. This prevent connections to be established with a cached session which was using the previous SSL_CTX. This patch also replace the ha_barrier with a thread_isolate() since there are more operations to do. The reg-test was also updated to remove the 'no-ssl-reuse' keyword which is now uneeded.
File list: - common.pem: PEM file which may be used by most of the VTC files.