Rework the execution model in burnin

This patch changes (significantly) the execution model in burnin:
  - for all runs, (almost) all instance mods in a single Burn* procedure
    are done as part of a job; so for example add disk, stop, remove
    disk, start are no longer done as separate jobs but as a single job
    consisting of four opcodes
  - for parallel runs, all Burn* procedures except the rename (which
    uses a single target name) run in parallel; before, only the
    creation was done in parallel
  - due to the single-job execution and also parallel execution, the
    logging messages are no longer happening synchronously with the
    execution, so they are more informative than an actual execution log

The end result is that burnin now tests properly multi-opcode jobs and
also tests all opcodes (except rename) for parallel execution.

Note: On a test cluster, parallelization reduces burnin time from 23m to
15m.

Reviewed-by: ultrotter