Postgres support for database and archive

[giesselmann]

Hi,
this is a draft PR for discussion to support postgres for both, db and job-archive.

Significant changes are:

• move archive code to internal (avoid circular imports)
• rename user table to users (keyword conflict in postgres)
• make squirrel statement builder an object (supports ? and $1 placeholder)

Still work in progress, need to commit archive backend code, need to solve issues with quoting of column names, maybe need to rename columns if there's no other way.