And rearrange files to work with the new layout for the finalized upgrader

Add a (temporary) key to %ticket_thread allowing quick lookups on the old_pk
column. Also, only fill the temporary table used for linking responses to
the migrated messages with messages.

When the `ostversion` column is removed from the %config table, the system
will immediately switch to database-backed sessions. Because the
administrator upgrading the system originally logged in with a file-based
session, it must be migrated to the database before removing the column.

Symbolic link to the upgrade patch for 1.6ST-1.7 upgrade - makes it easy, for devs, to indetify major patches