This change adds a new mechanism by which cluster members may identify each other whilst using X.509 based authentication.

Description of Linked Ticket

Define an X.509 extension that should explicitly be set on certificates when the clusterMembershipExtension configuration option is specified. All connecting clients that provide a certificate with that extension set to the value of clusterMembershipExtension will be treated as peer servers provided that the host server is configured to handle X.509 auth.