Use Fastly to do Compression

[dstufft]

Basically this will use Fastly to handle our compression rather than letting Warehouse do it.

This should reduce the amount of CPU our origin servers expend, but more importantly it means that we can do certain optimizations in Warehouse around conditional GET requests where we can skip executing the view if we know ahead of time that we can return a 304 Not Modified. Currently this is impossible in Warehouse because the compression tween needs to read the response body to handle the ETag on compressed responses.

This should roughly match the behavior of the existing compression tween, with the following differences:

• The existing tween only supports gzip, this supports gzip and brotli.
• The existing tween has some logic in it to skip compressing if the compressed result is larger than the uncompressed result, where I can't find any information what Fastly would do in this case.
• The ETags derived by the tween are likely going to be different then the ETags derived by Fastly.

This should work without code changes in Warehouse, because in vcl_miss and vcl_pass we modify the request that is going to be sent to the origin to remove the Accept-Encoding header, so Warehouse will only see requests that accept uncompressed content.