Split encoding/encryption on objects

[iglesiasbrandon]

We want to split encoding and encryption up for multiple reasons. Uploads can be thought of as transforming an incoming stream of bytes into a stream of encrypted segments, and then uploading each of those encrypted segments. We need to split those two bits apart so they can be used independently, even across a network. The first part of doing that is to figure out what interfaces the different components have.

Deliverable: Some interface descriptions for those two components.

Constraints:

• The return value from the splitting interface can be marshaled and sent over the network somehow to the inputs of the uploading interface (except for potentially the segment byte stream)
• It should not require reading a full segment into memory. Minimal memory usage would be best, though some lookahead is probably necessary for inline segments.

Notes:

• There is some code existing for doing some splits of a writer with a lot of details handled in the packages under https://github.com/storj/uplink/tree/upload-refactor/private/eestream

[kristaxox]

@zeebo poke for story point estimation :)