Cache connection object in an instance variable
Use `PG::Connection::quote_ident` to get quoted table name
Use raw_connection whenever perform sql queries
Use instance of `PG::Connection` as a connection
Instance of`PG::Connection` as a first-class citizen: pass it into a Writer so we don't use AR anymore.
TODO: Rewrite tests with this in mind.
Test `PostgresUpsert::Writer` ins specs but AR models. Also cache timestamp in the writer and use plain Ruby to calculate it.
Don't establish ActiveRecord::Base.connection in tests
Don't require fixture AR models in tests
Test AR extension separately.
Bump major version to 4.0.0 because PostgresUpsert::Writer#initialize signature has changed to accept an instance of PG::Connection and does not use AR anymore