Add foreign keys to shadow table during swap

[shayonj]

We setup the shadow table with the FKs (if any) so its structurally the same as the primary table since beginning

pg-osc/lib/pg_online_schema_change/functions.rb

Line 57 in 59f58c3

'ALTER TABLE %s add constraint %s %s',

During INSERT INTO (copying date from primary table to shadow) it can become a blocking operation as the new data is being validated by the FK. Rather, we can trust the FK on the primary table and add back the FKs on the shadow table prior to swap, so post swap they are structurally the same. We can accordingly extend validate_constraint to validate constraint on primary and referential tables.

[shayonj]

done in #67