Cache: increased cache header Vary and ETag lengths to 128.

This allows to store larger ETag values for proxy_cache_revalidate,
including ones generated as SHA256, and cache responses with longer
Vary (ticket #826).

In particular, this fixes caching of Amazon S3 responses with CORS
enabled, which now use "Vary: Origin, Access-Control-Request-Headers,
Access-Control-Request-Method".

Cache version bumped accordingly.