INTPYTHON-451 Add support for database caching

[WaVEV]

The MongoDB cache uses two different indexes:
Unique for the key and TTL index for the expiration date.

The Mongo's demon that deletes data from the collection is called every 60 seconds (if I am not mistaken) we have to deal with virtually deleted records (just filter if the record is expired)
So an extra exclude or include expired query are necessary.
Some scenarios to take in account:

1. counting avoiding expired thing
2. Do a set over an expired thing (must work as if the item isn't in the DB)
3. When Cull (capping routine is called) remove by expiration date.

fixes #229

[Jibola]

Why is the expiry set to 0 here? I know we talked about doing the filtering afterwards, but this is basically saying "expire immediately" is it not?

[Jibola]

If it's getting deleted almost instantly, but we also want to manually do the culling, wouldn't this just be better as a regular sorted index?

[WaVEV]

Yes, it expires when the time touch the expires_at value, I got the idea from here:
https://www.mongodb.com/docs/manual/tutorial/expire-data/#expire-documents-at-a-specific-clock-time

We also do culling. look at the method _cull

[WaVEV]

The idea is to handle different expiration times, so the set the expiration after 0 so we can chose whether we expires data

[timgraham]

It's not just the cache tests in Django's test suite that may need the table. If a Django project is using the Django test runner, what creates the table?

…

Message ID: <mongodb/django-mongodb-backend/pull/253/review/2646403770@ github.com>

[WaVEV]

It's not just the cache tests in Django's test suite that may need the table. If a Django project is using the Django test runner, what creates the table?
…
Message ID: </pull/253/review/2646403770@ github.com>

Ah ok, understood. I thought it was only used by unit test. Given that, yes, I should add it after the super

[timgraham]

You might compare the implementation with https://github.com/django-nonrel/mongodb-cache/blob/develop/django_mongodb_cache/backend.py. One thing I noticed is that your code doesn't consult router.db_for_write().

[WaVEV]

You might compare the implementation with https://github.com/django-nonrel/mongodb-cache/blob/develop/django_mongodb_cache/backend.py. One thing I noticed is that your code doesn't consult router.db_for_write().

Yeah, my bad. I created the method get_collection and forgot to handle the collection to write. I think I have to add the route test to the unit test.

[timgraham]

These two paragraphs replaced this one which was copied from the Django docs:

Unlike other cache backends, the database cache does not support automatic
culling of expired entries at the database level. Instead, expired cache
entries are culled each time add(), set(), or touch() is called.