Remove Storage#download

[janko]

When refresh_metadata plugin is used, UploadedFile#download using Storage#download causes bad performance when UploadedFile#download is called multiple times during metadata extraction, because in that case the same file will be downloaded from the storage multiple times.

add_metadata :foo do |io, context|
  io.download { ... } # downloaded from the storage (1st time)
end

add_metadata :bar do |io, context|
  io.download { ... } # downloaded from the storage (2nd time)
end

Moreover, the download that UploadedFile#refresh_metadata! started at the beginning of metadata extraction won't be reused. It will be reused for determine_mime_type and store_dimensions plugins, but for any custom metadata that uses UploadedFile#download (either directly or implicitly via Shrine.with_file) a whole new download will be started.

Storage#download existed for two reasons:

1. If Storage#open uses Down::ChunkedIO, calling UploadedFile#download the first time on an UploadedFile object would copy the remote content to two Tempfiles: the Tempfile that's returned and Down::ChunkedIO's internal Tempfile. Storage#download allows storage implementers to avoid the additional copy.

2. Some SDKs such as aws-sdk-s3 implement automatic retries only if the download destination is a Tempfile (like in S3#download), but not if it's a block (like in S3#open).

The 1st issue can be addressed by explicitly passing rewindable: false for storages that use Down::ChunkedIO and support :rewindable. Though it's not really a big deal anyway.

The 2nd issue should be fixed in the SDK. google-api-client already supports resumable downloads for any download destination, and fix for aws-sdk-s3 is in the works (aws/aws-sdk-ruby#1617)

I no longer believe Storage#download provides enough benefits to keep around. The performance issues with refresh_metadata plugin and custom metadata are a great reason to throw it away. An added benefit is that storages now have one less method to implement.

Partially addresses #329