Add `actor` table and code to start using it

Authored by Anomie on Sep 12 2017, 5:12 PM.


Add actor table and code to start using it

Storing the user name or IP in every row in large tables like revision
and logging takes up space and makes operations on these tables slower.
This patch begins the process of moving those into one "actor" table
which other tables can reference with a single integer field.

A subsequent patch will remove the old columns.

Bug: T167246
Depends-On: I9293fd6e0f958d87e52965de925046f1bb8f8a50
Change-Id: I8d825eb02c69cc66d90bd41325133fd3f99f0226