patched http custom client concurrent map writes

• Patched duplicate interface method with go1.11

• Added WriterProvider interface
• Changed NewWriter signature (breaking change)
• Changed Download signature (breaking change, use Open instead)
• Changed DownloadWithURL signature (breaking change, use OpenURL instead)
• Renamed Storager.Download to Storage.Open (breaking change)
• Renamed storage.Downloader to storage.Opener (breaking change)
• Optimized zip/tar walk streaming
• Added file.NewWriter

• Added asset.NewGzFile testing utility

• Added service.Open method
• Added service.OpenURL
• Both Open use original Download functions, for backward compatibility it may stay like this for a while, however ideally Download method should return []byte

• removed viant/toolbox dep

• Added option.Region (for cloud storage)
• Added option.ServerSideEncryption

• Patched mem fs race condition

• Patched precondition check on mem fs

• Added NewWriter(ctx context.Context, URL string, mode os.FileMode, options ...storage.Option) io.WriteCloser