Deeply freeze Config except Buffers and Readable streams

[mhagmajer]

Following up on #146 which got reverted, I'm using the same approach but while also avoiding freezing Readable streams. Added unit tests from the original PR and also another one to cover the Readable stream case.

I created an external Node application that was using SDK with this change applied to perform several common tasks:

• fetching a folder
• getting retention policy assignments
• getting files under retention for an assignment
• getting file versions under retention for an assignment
• deleting a file
• uploading a file (here's where Readable stream was used)

Closes #142.