LU-6714 llog: test plain llog more throughly

LU-6714 llog: test plain llog more throughly

Modify llog test 3 to check more aspects of its
functionality:
- search the record by index
- check all records are processed
- check modification in-place
- check that processing is seeing concurrently added
  new records
- check the correctness of lgh_cur_offset and lgh_cur_idx

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I11a7c6474a7420393987a964e9112d61263bdc81
Reviewed-on: http://review.whamcloud.com/15835
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>