Don't cache metadata in GCS connections
Currently if you access a bucket, change the contents, and try to read a new file w/ e.g. `read_parquet`, you see a `FileNotFoundError`; restarting Python fixes this. IMO since the `gcsfs` connection is hidden from the user, caching metadata doesn't really make sense (how would you even go about clearing this manually?).

@martindurant seem ok to you?