Use Redis as storage backend instead of database

[alextreme]

As discussed with Sergei, using Redis (or the Django cache backend) for storing the log-outgoing-requests OutgoingRequestsLog model-data would have a number of advantages:

• We could add a TTL, that the data would automatically expire
• We avoid a database call to Postgres when storing data
• We avoid having to deal with transactions when an exception is raised
• Clearing the cache means clearing out the requests/responses logs.

I've thought a bit about this as it sounds good, but there are a few drawbacks in my eyes:

• We won't be able to use FKs/M2Ms in the future (that's not currently implemented or expected, but knowing timelinelogger it may be useful to be able to link requests to a User in the future -> then again this could also be done by storing a username)
• We won't be able to make use of database validation? -> we can reuse validators already in Django for the specified fields
• We won't be able to make use of the Django admin interface for looking through the data (especially the search and filters is proving useful) -> this would be a non-significant amount of code & work to create a workable interface. Third-party Django redis-admin packages are more low-level and don't seem like interfaces I'd want to show to an admin. But it could be that I overlooked something useful.

Afaik it's also not possible to use Redis as a ORM storage backend... although it would be interesting to see what's possible I'd prefer not to do anything too hacky with a shared library.