Move db-level SQL statements to the file which implements the storage interface

*This issue was originally created by [**@kegsay**](https://github.com/kegsay) at .*

In several places for postgres, we `CREATE SEQUENCE ...`. We do this for every table which uses the sequence, partly for clarity and partly so we don't assume someone else has done it for us.

Instead, we should put these database-level SQL queries in a dedicated section, rather than in each `_table.go`.