download to stdout does not block when pipe is full

[Finlay Thompson]

When using swift to download directly to a linux pipe the swift process can run out of memory.

The use case is downloading a large fastq file (60Gb) and streaming it directly into an application that consumes the pipe.

The problem is that process that consumes the pipe can't keep up, and the pipe fills up. At this point the swift client should pause until it can write again, instead of buffering the file in memory.