changed the behavior of the swapping of the temp table in _s3fs_refresh_cache. instead of dropping and renaming tables, it now deletes all the rows from the original table and then inserts all the rows from the temp table. this allows the entire update to take place within a transaction with full support for rolling back.
changed db_delete to db_truncate, which is more 'correct' in this context.
switched back to db_delete, because apparently db_truncate does an implicit commit also. fixed the select query so that it would return the correct fields.
Issue #2163711: Improved the README.

The post-install configuration instructions were not as clear as I’d
intended, so I re-formatted the entire file to make each section pop out.
Issue #2166757: Users may now set up AWS Credentials on the s3fs settings page.

Settings applied using the existing settings.php mechanism will override any
changes made on the settings page.
Issue #2168589: Fixed folder existence check for paths with a trailing slash.
REFACTOR: Updated some of the settings documentation.

I found out that S3 officially doesn’t support serving torrents through
pre-signed URLs, so I tweaked the way that the torrent code gets run in
relation to the pre-signed and save-as code, and re-wrote the documentation
to make this clear to users.
REFACTOR: Code tweaks that should have been in the previous commit.
Issue #2177523: Added a test for is_dir().