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
Merge pull request #10172 from senny/10098_backport

Backport: Correctly parse bigint defaults in PostgreSQL