Add in-depth logging for background jobs

[pietroalbini]

We should add more logging in the job executor (swirl), to record this information:

• A job starts executing, with the name and all the parameters of such job.
• A job finishes executing, with its output and execution time.

[jtgeibel]

I think this is addressed by #2231 (for git index operations) and #2066 (for update_downloads and database_dump jobs). The logging is done by the individual jobs and not directly by the executor, but I believe the current level of logging is now sufficient for these background jobs.