Replace `layer.objectWritePayload` method with `initObjectPayloadReader`
which returns `io.Reader` of the object payload. Copy payload data to
the parameterized `io.Writer` in `layer.GetObject`. Remove `io.Pipe`
from `CompleteMultipartUpload` implementation and build analogue of
`io.MultiReader` for the part list.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
8fb3835250