Backport: Correctly parse bigint defaults in PostgreSQL

Backpost #10098.

Conflicts:

	activerecord/CHANGELOG.md
	activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
	activerecord/test/cases/schema_dumper_test.rb

Had to do some minor adjustments to get it working with `3-2-stable` but nothing severe.