S3BinaryCacheStore: Support compression of narinfo and log files

You can now set the store parameter "text-compression=br" to compress
textual files in the binary cache (i.e. narinfo and logs) using
Brotli. This sets the Content-Encoding header; the extension of
compressed files is unchanged.

You can separately specify the compression of log files using
"log-compression=br". This is useful when you don't want to compress
narinfo files for backward compatibility.