LU-15481 llog: Add LLOG_SKIP_PLAIN to skip llog plain

LU-15481 llog: Add LLOG_SKIP_PLAIN to skip llog plain

Add the catalog callback return LLOG_SKIP_PLAIN to conditionally skip
an entire llog plain.

This could speedup the catalog processing for specific usages when a
record need to be access in the "middle" of the catalog. This could
be usefull for changelog with several users or HSM.

This patch modify chlg_read_cat_process_cb() to use LLOG_SKIP_PLAIN.
The main idea came from: d813c75d ("LU-14688 mdt: changelog purge
deletes plain llog")

**Performance test:**

* Environement:
2474195 changelogs record store on the mds0 (40 llog plain):
mds# lctl get_param -n mdd.lustrefs-MDT0000.changelog_users
current index: 2474195
ID    index (idle seconds)
cl1   0 (3509)

* Test
Access to records at the end of the catalog (offset: 2474194):
client# time lfs changelog lustrefs-MDT0000 2474194 >/dev/null

* Results
- with the patch: 	real    0m0.592s
- without the patch:	real    0m17.835s (x30)

Signed-off-by: Etienne AUJAMES <etienne.aujames@cea.fr>
Change-Id: I887d5bef1f3a6a31c46bc58959e0f508266c53d2
Reviewed-on: https://review.whamcloud.com/46310
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>