Without this buffering, the compressor was outputting 64 bytes at a
time to the HTTP stream, which was resulting in absurdly small chunk
sizes and a lot of extra overhead. The buffering restores the chunk size
to 32768 bytes, which matches the behavior with 1.8.2.
Times pushing to a local registry:
1.8.2: 0m18.934s
master: 0m20.564s
master+this commit: 0m17.593s
Fixes: #17038
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>