Disable triggers and hold access when running ScanCore's update_db()

Set a 'variables' entry to mark an ongoing DB update and have connect_to_db() check and, if set, loop until the variable is released (with a sane timeout).

Then setup the ScanCore schema to allow the Striker user to set/release 'session_replication_role'. See C. Keane's reply in this thread: https://forums.aws.amazon.com/thread.jspa?messageID=561509.

Start at update_db() and connect_to_db(), then inspect the ScanCore schema and the referenced AWS forum reply. The work is done when an update marks the database as busy, connections wait with a sane timeout, and the Striker user can set and release session_replication_role as required.

Written by the indexing model from the issue text.