Fix port script so that it can be run again after failure. (#8755)

If the script fails (or is CTRL-C'ed) between porting some of the events table and copying of the sequences then the port script will immediately die if run again due to the postgres DB having inconsistencies between sequences and tables.

The fix is to move the porting of sequences to before porting the tables, so that there is never a period where the Postgres DB is inconsistent. To do that we need to change how we port the sequences so that it calculates the values from the SQLite DB rather than the Postgres DB.

Fixes #8619