Optimize total count of feed when loading activities in user dashboard.

[lunny]

Two SQLs are very slow when action table have over 5M records.

database duration=1.8881s db.sql="SELECT created_unix DIV 900 * 900 AS timestamp, count(user_id) as contributions FROM `action` WHERE user_id=? AND act_user_id=? AND (created_unix > ?) GROUP BY timestamp ORDER BY timestamp"

database duration=1.5408s db.sql="SELECT count(*) FROM `action` WHERE (user_id = ?) AND (is_deleted = ?)"

This will cache the count for the first loading or when the activities changed.

[delvh]

created_unix DIV 900 * 900

Wait what? Why is this query so weird in the first place?

[lunny]

created_unix DIV 900 * 900

Wait what? Why is this query so weird in the first place?

This was introduced by #15252