Architecture for "Your activity stream"

As a followup to #1710688: Activity stream & Email notification system.

Not explicity shown but suggested in the Commons interractive prototype is the "Your activity stream" functionality, a listing of activity that corresponds to the "following" flag for nodes, users, terms and groups.

This issue is for discussion about how to best record and display the on-site activity stream.

All message types have the uid property, which corresponds to the acting user. Messages also entity reference fields for target users and target nodes that the message is about.

"My activity stream" - A single listing of message entities.

A couple of approaches to displaying this listing.

A) A single query generated by a View with many JOINs and OR clauses

. To do this, we'd need to be:

- Joining message entity user property to flag:
The message entity UID property matches a user entity flagged by the current user

- Joining message entity reference field to flag:

One of the message referenced target users is flagged by the current user
The referenced target node entities are flagged by the current user

- Joining taxonomy term or group membership data to message entity reference field to flag:

The referenced target node entities are in a group flagged by the current user
The referenced target node entities contain a term flagged by the current user

Approach A seems pretty ugly because:
- We'd be combining multiple JOINs & OR clauses with an ORDER BY clause, which in preliminary EXPLAINs in MariaDB 5.2.8 will include temp tables, even when add an index on {message}.timestamp.
- This would require adding multiple Views relationships which prevent the View from having any results, even without adding filters, so it's not clear that this approach would even display the desired information.

B) Multiple queries passed into a single Views filter

1) Query for NIDs, UIDs, and TIDs that the user is following based on what they have flagged.
2) Then, find a list of message IDs related to those entities.

(We'd be examining these tables: node, message, target_nodes, target_users, og_membership, field_data_field_topics)

And looking for data such as:
- Where target_nodes in (1 ,2, 3, 4)
- Where target_users in (4,5 6)
- Where message.uid in (4,5,6)
- node joined to field_data_field_topics (or similar) (where in term ids)
- node joined to og_membership where in (group ids)

3) Finally, render the list of messages and apply the order clause.

This could be achieved with Views via a custom filter that separately runs its own queries, and generates a simple IN() query containing list of message IDS. Add an ORDER BY and LIMIT clause and the view is done.

C)Alter the Views query to do a single query without explicit mids and improve indexes that will prevent writing to disk

Below is the output from an EXPLAIN on a simple query generated by a view that connects the referenced target nodes field to the commons_follow_node flag. We could investigate optimizations that would make queries such as this one -- but with many more joins for the various types of flags more scalable & performant.