Improvements to how group total_member_count is refreshed on membership events

- Use an actual member count when refreshing, rather than incrementing
- Centralize all of the updating in the database methods, rather than having
  the updates spread throughout the groups component

Also adds integration tests for total_member_count as relates to the groups_*
functions.

See #5018

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7182 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Better cache busting of user group counts.

See #5018

Props r-a-y

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7183 cdf35c40-ae34-48e0-9cc9-0c9da1808c22