Simplify data verification with HashReader. (#5071)

Verify() was being called by caller after the data
has been successfully read after io.EOF. This disconnection
opens a race under concurrent access to such an object.
Verification is not necessary outside of Read() call,
we can simply just do checksum verification right inside
Read() call at io.EOF.

This approach simplifies the usage.

cmd/streaming-signature-v4.go

@@ -164,7 +164,7 @@ var errMalformedEncoding = errors.New("malformed chunked encoding")
 //
 // NewChunkedReader is not needed by normal applications. The http package
 // automatically decodes chunking when reading response bodies.
-func newSignV4ChunkedReader(req *http.Request) (io.Reader, APIErrorCode) {
+func newSignV4ChunkedReader(req *http.Request) (io.ReadCloser, APIErrorCode) {
 	seedSignature, region, seedDate, errCode := calculateSeedSignature(req)
 	if errCode != ErrNone {
 		return nil, errCode
@@ -242,6 +242,10 @@ func (cs chunkState) String() string {
 	return stateString
 }
 
+func (cr *s3ChunkedReader) Close() (err error) {
+	return nil
+}
+
 // Read - implements `io.Reader`, which transparently decodes
 // the incoming AWS Signature V4 streaming signature.
 func (cr *s3ChunkedReader) Read(buf []byte) (n int, err error) {