Remove Blob conversion from ImageToTarballTranslator and TarStreamBuilder

[chanseokoh]

I am going to do a serious of refactoring to our repo (not about our API visibility but about making our codebase healthy), and this is the first step. I have many more PRs to come.

When I first came to the repo, Blob was a very confusing class. As much as the name says, it is a very general class and can wrap up any object, anything that holds any information, or anything that may potentially be written to somewhere. It often disguises the actual object to the point of not being able to get what it really is. To me, often Blob looks like the Java Object class.

Another problem is the abuse of Blob to write something to OutputStream. With Blob having toWrite(OutputStream), examples of prevalent abuse in our codebase are

1. Convert anything to Blob blindly for the sake of writing to OutputStream.
2. Convert anything to Blob blindly to compute a digest of a content. (Blob.writeTo() returns a digest.)

Currently, ImageToTarballTranslator (which effectively represents a tarball for an Image) is doing a convoluted way of writing it to a file or a Docker daemon:

ImageToTarballTranslator --> Blob --> write to an output stream

In this PR, I'm making this straightforward and intuitive by

1. just making ImageToTarballTranslator write to an output stream; and
2. decoupling Blob from ImageToTarballTranslator.

Also, DockerClient.load() accepting Blob seems is too general and obscure, which doesn't look very different from load(Object anything).

[chanseokoh]

Similarly to ImageToTarballTranslator, I also removed Blob from TarStreamBuilder, which it had toBlob() only to do toBlob().writeTo(out).