Current pipelining to stdout is synchronous so we don't have any
advantage from using --parallelism flag. This change adds buffer
while writing to stdout. Each part is first read into the buffer
and flushed only when all data was read from this part.
https://github.com/storj/uplink/issues/105
Change-Id: I07bec0f4864dc4fccb42224e450d85d4d196f2ee
