Refactor streaming signatureV4 w/ state machine (#2862)

* Refactor streaming signatureV4 w/ state machine

- Used state machine to make transitions between reading chunk header,
  chunk data and trailer explicit.

* debug: add print/panic statements to gather more info on CI failure

* Persist lastChunk status between Read() on ChunkReader

... remove panic() which was added as interim aid for debugging.

* Add unit-tests to cover v4 streaming signature

cmd/fs-v1.go

@@ -387,8 +387,9 @@ func (fs fsObjects) PutObject(bucket string, object string, size int64, data io.
 		var bytesWritten int64
 		bytesWritten, err = fsCreateFile(fs.storage, teeReader, buf, minioMetaBucket, tempObj)
 		if err != nil {
+			errorIf(err, "Failed to create object %s/%s", bucket, object)
 			fs.storage.DeleteFile(minioMetaBucket, tempObj)
-			return ObjectInfo{}, toObjectErr(traceError(err), bucket, object)
+			return ObjectInfo{}, toObjectErr(err, bucket, object)
 		}
 
 		// Should return IncompleteBody{} error when reader has fewer