Fix race condition during write to internal RBTDB.
    
    RBTDB implementation allows to open only one RBTDB instance
    for writing at the same time. This patch adds mutex to
    newversion() implementation in ldap_driver.c.
    
    See comments around ldapdb_t, newversion() and closeversion().
    
    Signed-off-by: Petr Spacek <pspacek@redhat.com>