SYSDB: Rework sysdb_cache_connect()
    
    As sysdb_cache_connect() has two very specific use cases (connect to the
    cache and connect to the timestamp cache) and each of those calls have a
    predetermined/fixed sets of values for a few parameters, let's try to
    make the code a bit simpler to follow by having explicit functions for
    connecting to the cache and connecting to the timestamp cache.
    
    Macros could be used as well, but I have a slightly preference for
    having two new functions instead of macros accessing internal parameters
    of the macro's parameter.
    
    Related:
    https://fedorahosted.org/sssd/ticket/3128
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>