Modify bp_update_meta_cache() for new metadata cache schema

Now that we are using the WP metadata API, our cache keys have changed, and
the logic for priming meta caches must mirror these changes.

See #4551

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7843 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Bust group object cache when updating groupmeta

This is important because some group metadata gets stored with the group
object (like last activity)

See #4551

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7844 cdf35c40-ae34-48e0-9cc9-0c9da1808c22