Fastpath writes to disk

[LDiazN]

With our current setup the API in the monolith is writing report data to disk with the /report/{id} endpoint while also sending the same data to the fastpath.

This data is meant to be processed by the api uploader.

However since we intend to move this setup to AWS, we need a different approach, see #968

This PR will move the file-writing part to the Fastpath once the measurement is received. This is necessary because the ooniprobe service is unable to share disk with the EC2 machine running fastpath and api uploader

The file-writing feature is under a feature flag and deactivated by default. To activate it we add this in fastpath.conf

[DEFAULT]
...
write_to_disk = true

It will also add the config parameter msmt_spool_dir to the configuration file. It will only be parsed when write_to_disk == true

depends on #969
closes #968

[hellais]

Thanks for putting this together it looks very good. I have left a few comments for things to change. Once addressed we can move it forward.

[hellais]

LGTM!

[hellais]

Next steps to get this deployed in production:

1. Bump version inside of https://github.com/ooni/backend/blob/master/fastpath/debian/changelog to 0.87
2. Test deploy 0.87 on backend-hel to make sure the fastpath without write-to-disk enabled still works as intended
3. Land this into master and deploy the master branch onto dev
4. Do some testing with ooniprobe and miniooni coordinating with @DecFox
5. Make a plan to deploy it into production during a specific week (ideally starting deployment on Monday)