Download outputfiles in background

[sverhoeven]

Given a job is completed on a remote system without a shared file system.
The output files of the job must be copied back to the file system which can be seen by the bartender service.

In #17 the copying of output files is done during handling of GET /api/job/<jobid> or GET /api/job HTTP requests.
This works fine for small amounts (<1Mb) of bytes to copy, but for big amounts (~40Gb) this will cause timeouts on the HTTP requests.

It would be nice if we could use https://fastapi.tiangolo.com/tutorial/background-tasks or use some file copying queue.

A new job state should be introduced like staging_out to signal that copying of files is happening for a job.
The staging_out job state can also be used to prevent copying multiple times This is not a big issue as files should not change after job completion.

For uploading input files to a remote file system the background task is already used.

[sverhoeven]

Implemented in #33

[sverhoeven]

Doh #33 was merged into wrong branch arq

[sverhoeven]

Fixed in PR #62 and #61