LDAP: sdap_id_ctx might contain several connections
    
    With some LDAP server implementations, one server might provide
    different "views" of the identites on different ports. One example is
    the Active Directory Global catalog. The provider would contact
    different view depending on which operation it is performing and against
    which SSSD domain.
    
    At the same time, these views run on the same server, which means the same
    server options, enumeration, cleanup or Kerberos service should be used.
    So instead of using several different failover ports or several
    instances of sdap_id_ctx, this patch introduces a new "struct
    sdap_id_conn_ctx" that contains the connection cache to the particular
    view and an instance of "struct sdap_options" that contains the URI.
    
    No functional changes are present in this patch, currently all providers
    use a single connection. Multiple connections will be used later in the
    upcoming patches.