Normalize cache priming in BP_Activity_Activity::get_activity_data()

No need to reproduce logic in bp_get_non_cached_ids(). This brings the caching
strategy in line with other components.

See #5434

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8075 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Improved cache invalidation when posting activity comments

- Because activity comment caches are stored only with the top-level parent,
  we should be invalidating the bp_activity_comments cache for that item, not
  for the immediate parent
- When a new comment is created, clear the bp_activity cache for each
  ancestor

See #5434

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8076 cdf35c40-ae34-48e0-9cc9-0c9da1808c22