feat: enalbe councrrent oss backsource by set contentLength
pieceManager calls DownloadSource() to download source objects.
DownloadSource() firstly connect to source and get the response from
source. Then, DownloadSource() checks the "ContentLength" value of the
response. If it larger than 0, pieceManager will download the object by
multiple threads.
Unfortunately, oss client doesn't set ContentLength value, even though
ContentLength can be set by the oss.GetObjectResult.Response.Headers.
So let's set response a ContentLength value to enable concurrent
downloads from oss.
Signed-off-by: hiigao <hii_gao@hotmail.com>
eebca2416a