LU-5038 llog: do not scan for empty llogs at initialization

LU-5038 llog: do not scan for empty llogs at initialization

in some cases the catalog can contain lots of non-empty llogs
(e.g. robinhood isn't running all the time and this lead to
many alive plain llogs) and scanning this in the context of
mount can take a lot. instead we can do this during normal
llog processing.

Change-Id: I6c23b02b4a9733f80599314d37ffac6d2ee57001
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/10308
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>