GetRangedHttpReader some http server doesn't support "Range" header, so this function read readCloser with whole data, skip offset, then return ReaderCloser.

NewBuf is a buffer that can have 1 read & 1 write at the same time.

NewMultiReadCloser to save memory, we re-use limited Buf, and feed data to Read().

RequestHttp deal with Header properly then send the request.

ServeHTTP replies to the request using the content in the provided RangeReadCloser.

DefaultDownloadConcurrency is the default number of goroutines to spin up when using Download().

DefaultDownloadPartSize is the default range of bytes to get at a time when using Download().

DefaultPartBodyMaxRetries is the default number of retries to make when a part fails to download.

LimitedReadCloser wraps a io.ReadCloser and limits the number of bytes that can be read from it.