LU-14158 mdc: process changelogs_catalog from the oldest rec

LU-14158 mdc: process changelogs_catalog from the oldest rec

The chlg_load use the LLOG_CAT_FIRST to process changelogs. This
values will process record in the catalog always starting with index
0 to the newest record. So when catalog reach the end of indexes and
when records are saved at the begining of catalog, the
llog_cat_process will ignore records at the end.

This patch change the "startcat" value LLOG_CAT_FIRST to 0 to scan
the catalog from the oldest record to the newest.

Fixes: e2150028 (LU-12553  mdc: polling mode for changelog reader)
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Change-Id: Ib00a4373d87fd63bfb5d05731f1fdbd286c0b6f5
Reviewed-on: https://review.whamcloud.com/40786
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>