Select activities cache is not invalidated after a change

[ilicfilip]

There is a helper method for fetching activities from our custom DB table and it uses wp_cache_set and wp_cache_get functions in order to cache the results. Hashed query arguments are used as a cache key.

The problem is that cache is not invalidated after a change was made to the to the activities, for example:

1. Select all activities for a post
2. Insert new activities for the post
3. Run the same query as in 1)

Second query will return results results which were cached in 1), omitting the change which was made in 2)
Since cache is not persistent it is not a problem for now, but it can result in some weird behaviour in the future and it definitely needs to be fixed.