Optimize heatmap query

[lunny]

When there are over 5M records on action table, the heatmap on dashboard is very slow as below SQL.

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" 

This PR add a new index for action table with columns user_id, act_user_id and created_unix so that this query will become about 6 times faster than before.

[silverwind]

are those index names consistent with previous indexes? maybe better to spell out the column names wholly.

[lunny]

are those index names consistent with previous indexes? maybe better to spell out the column names wholly.

ce57b3b will follow previous index naming pattern.

[silverwind]

I mean why not use act_user_id_created_unix_user_id instead of au_c_u? Note I don't know who other indexes were named.

[lunny]

I mean why not use act_user_id_created_unix_user_id instead of au_c_u? Note I don't know who other indexes were named.

All other indexes in this table uses a first character of the columns in this index. au == act_user_id, c == created_unix and u == user_id.