Fixed too much memory usage for s3 streaming

Added memory usage tests as well for streaming uploads and downloads.  The actual snippet of code that I changed was in my original pull request before I rebased off of develop.  I added test for memory usage for both streaming uploads and streaming downloads so that this does not happen again.

Fixes https://github.com/aws/aws-cli/issues/923

cc @jamesls @danielgtaylor 

I meant in the python process running our integration tests.  Especially on various CI servers that don't have lots of memory, we don't necessarily want to create a 200mb string in process when we can just easily avoid it.  I think the most we buffer in memory was previously 10MB so this is a pretty big jump up to 200mb.