Introduce a PK/id field on audit table for ordered reads

[shayonj]

There is a potential edge case right now when reading from audit table. Since the read uses the primary_key of the primary table, it may be out of order if two updates happen for the same row.

Its rather better to have a dedicated id field (PK) on the audit table, that way when reading we can have ordered entries w/o using a timestamp field

pg-osc/lib/pg_online_schema_change/replay.rb

Line 32 in 4e48849

SELECT * FROM #{audit_table} ORDER BY #{primary_key} LIMIT #{PULL_BATCH_COUNT};

This field name, needs to be non conflicting (with primary table) as well. Similar to: #47