Support object cache in groups_get_group(), and use it instead of directly instantiating BP_Groups_Group

[boonebgorges]

See #3758.

BP functions that directly instantiate BP_Groups_Group (for instance, bp_the_topic_object_avatar()) can be wasteful when run many times on a single load. Not to mention that we have no support for persistent caches for objects that, for the most part, are quite static over time.

The attached patch is a first pass at cache support. It does the following:

• Adds support for the WP cache to groups_get_group()
• Swaps out most existing instantiations of BP_Groups_Group with groups_get_group()
• Adds a bit of cache-busting logic to methods of BP_Groups_Group, especially for those using object cache plugins.

In my testing, this patch can shave up to 50 queries off of a single page load!

Thoughts or feedback?

[boonebgorges]

(In [5431]) Adds WP cache support to groups_get_group() and switches internal instantiations of BP_Groups_Group out for calls to groups_get_group() where possible. Fixes #3770