Add opcode execution log in job info

This patch adds the job execution log in “gnt-job info” and also allows
its selection in “gnt-job list” (however here it's not very useful as
it's not easy to parse). It does this by adding a new field in the query
job call, named ‘oplog’.

With this, one can get a very clear examination of the job. What remains
to be added would be timestamps for start/stop of the processing for the
job itself and its opcodes.

Reviewed-by: imsnah