Refine `ExternalStorage` interface.

[SunRunAway]

The interface ExternalStorage force us to save the entire file into memory when reading or writing.

// ExternalStorage represents a kind of file system storage
type ExternalStorage interface {
	// Write file to storage
	Write(ctx context.Context, name string, data []byte) error
	// Read storage file
	Read(ctx context.Context, name string) ([]byte, error)
	// FileExists return true if file exists
	FileExists(ctx context.Context, name string) (bool, error)
}

Suggest a new interface

// ExternalStorage represents a kind of file system storage
type ExternalStorage interface {
	// Write file to storage
	NewWriter(ctx context.Context, name string) (io.WriteCloser, error)
	// Read storage file
	NewReader(ctx context.Context, name string) (rc io.ReadCloser, size int64, err error)
	// FileExists return true if file exists
	FileExists(ctx context.Context, name string) (bool, error)
}

[SunRunAway]

We can start to do it when #102 and #103 are merged.

[SunRunAway]

@gregwebs @overvenus @kennytm @tennix @DanielZhangQD

[overvenus]

A minor question, why do we need to return a size in NewReader?