Init is public so it can be called from plugin_auth_ldap.go (go/cmd/vtgate).

AuthServerLdap implements AuthServer with an LDAP backend.

ClientImpl is the real implementation of LdapClient.

LdapUserData holds username and LDAP groups as well as enough data to intelligently update itself.

ServerConfig holds the config for and LDAP server * include port in ldapServer, "ldap.example.com:386".

Client provides an interface we can mock.