Support updating existing records in postgres

[craigedmunds]

When running a simple app, moving data from mongo to postgres, if there's a unique constraint, and records exist already, it ends up failing.

Update the postgres adapter to support the "ON CONFLICT DO UPDATE" syntax seems the best way to prevent this.

Have a basic implementation that needs refining (I'm not v proficient in Go!)

[craigedmunds]

The basic implementation:

• Needs tests
• Needs to act on primary keys not
• Shouldn't be as noisy logging!
• The idea of GenerateUpsertStatement is that it can be shared between update and insert, should refactor update