Sometimes consumers of Seek() might use it for purposes other than
chaging the current read/write offset. For example, using whence =
io.SeekCurrent to get the current seek position. However, minio-go
invalidates the underlying HTTP stream unconditionally when Seek() is
called, resulting in massive performance degradation in these scenarios.

This commit avoids these issues by only reopening the stream if the seek
position is actually changed.