Check caching of `SHOW max_identifier_length`

The test [`test_payload_name_on_eager_load`](https://github.com/rails/rails/blob/90a1eaa1b30ba1f2d524e197460e549c03cf5698/activerecord/test/cases/instrumentation_test.rb#L48) would fail, because we ware making a schema query `SHOW max_identifier_length`. We should verify why this query is made, and if it there a pg cache mecanism that we missed in our adapter. 