Add helper function which accepts container and object identifiers and
returns object instance read to the memory.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Make `ObjectReader` / `ObjectRangeReader` to track number of bytes read
and return:
* `io.ErrUnexpectedEOF` if the stream finished by the server before
the last byte of the payload was read.
* an error if more object payload size bytes was read.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
