Implement an optimized version of delete_many() for the db cache backend

[oliver]

Now delete_many() in core.backends.base.BaseCache is as follows.

    def delete_many(self, keys, version=None):
        """
        Delete a bunch of values in the cache at once. For certain backends
        (memcached), this is much more efficient than calling delete() multiple
        times.
        """
        for key in keys:
            self.delete(key, version=version)

And cache.backends.db does not implement delete_many()
So if i retrieve 100 entries from the db cache, 100 queries are executed instead of just one
It should be fixed

This ticket is similar to https://code.djangoproject.com/ticket/29584

[oliver]

​https://github.com/django/django/pull/10258

[Carlton Gibson]

This seems reasonable given #29584.
(Possibly merge the two tickets?)

[oliver]

​https://github.com/django/django/pull/10257

​I rebased ​https://github.com/django/django/pull/10257 's branch onto ​https://github.com/django/django/pull/10258 's branch.
So i think that just ​https://github.com/django/django/pull/10257 needs to be merged.

[Tim Graham <timograham@…>]

In abd0ad76:

Fixed #29626, #29584 -- Added optimized versions of get_many() and delete_many() for the db cache backend.