Global cache group improvements

[johnjamesjacoby]

BuddyPress currently relies on the 'bp' cache group in many places where a more specific group would be more appropriate.

For example

• The entire Blogs component uses this group, instead of its own bp_blogs group.
• bp_activity_sitewide_front uses bp rather than bp_activity
• Friends uses bp for member relationships.
• Groups uses bp for total group counts and user memberships.
• XProfile uses bp for groups and visibility levels.

I think most of these would benefit from having/using their own global cache group.

[johnjamesjacoby]

Related #5733.

[r-a-y]

+1. This is long overdue!

[r-a-y]

Going to mark this as a duplicate of #6341 as that ticket has some commits to it and is essentially the same issue.

Will copy this ticket description over.