LU-15645 obdclass: llog to handle gaps

LU-15645 obdclass: llog to handle gaps

due to old errors an update llog can contaain gaps in index.
this shouldn't block llog processing and recovery. actual
gaps in transaction sequence should be catched by VBR.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I11ec817e356f9658118c34706ef3a533e7faba83
Reviewed-on: https://review.whamcloud.com/46837
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>