introduce `conn.data_source_exists?` and `conn.data_sources`.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
	activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
	activerecord/lib/active_record/connection_adapters/schema_cache.rb
	activerecord/lib/active_record/schema_dumper.rb
	activerecord/lib/active_record/type_caster/connection.rb
	activerecord/test/active_record/connection_adapters/fake_adapter.rb
	activerecord/test/cases/adapters/postgresql/schema_test.rb