Clean up merging a bit more.

* Update merged jobit count in the dataset stats update function.
* Remove unions that are querying the same for process and merge jobs.
* Don't update jobits to merged anymore.
Make JobHandler internals “protected”.
Merge merging jobs into the data processing.

All jobs (including merges) have the following status progression now:

    ASSIGNED → SUCCESSFUL → MERGING → MERGED

Merging jobs are drawn from all jobs marked `SUCCESSFUL`, and only those
jobs with that status should be considered for further processing, e.g.,
publication.  Merging stops under the following condition:

* All jobits are processed
* No jobs are currently running
* Failure to create at least one merging job

The state of jobits is **not** altered during merging.  Merging only
considers jobs.

Leftover bits:

* Have merge jobs have a number of jobits corresponding to the lumis they
  process, not the jobs
* Fix plotting to mostly show processing jobs
* Cross-check everything thoroughly

Fixes #51 for now.