chore(dao) modify postgres for bdr compatibility

[AlexBloor]

Summary

This change helps support compatibility with BDR.
Kong performs postgres database migrations on start-up.
Postgres logs an error when postgres.lua calls ALTER TABLE … DEFAULT.
Per the BDR docs, ALTER TABLE ... DEFAULT is not supported by BDR.
However, it can be rewritten into a short sequence that is supported.
This commit replaces the one command with the short sequence.

http://bdr-project.org/docs/0.9/ddl-replication-statements.html

Full changelog

• [Implement restructured postgreSQL commands in kong/dao/migrations/postgres.lua]

Issues resolved

Fix #2671

[p0pr0ck5]

LGTM, though I'm not sure if this should target master or next (typically migrations are only released on next, but since this is just a minor update, not a new migration, I don't know if we need to follow this pattern here). @Tieske thoughts?

[thibaultcha]

This is not a new migration, but an improvement on a current one, so master is the right candidate.