Deleted feature sets not removed from cache

[lrouquette]

Steps:

1. Create a feature set
2. Retrieve the list of sets from cache endpoint e.g. /sets (presumably that list is now cached)
3. Delete the feature set
4. After a reasonable amount of time, retrieve the list of sets again (same as 2.)

Result: the deleted feature set is still in the list.

This is a pretty low severity issue in terms of functionality, but it does mean that the /sets API doesn't return the same data on the cache vs. the origin, which could be significant.