Prevent to store in memory all layers

[ChrisCinelli]

On my machine, for large images, the RAM is more a constraint than the CPU.

The whole layers are copied in memory. I tried to fixing this while I was working on #103 .
This is what I did : ChrisCinelli@8e4ea7c#diff-da07f373046a0a24e67b18699dc15a70
but I had to remove the concurrency since the reader is not thread safe.

By the way, I also think you have a problem with accessing to the layerMap in processLayerTar at the moment.

[ChrisCinelli]

Implemented on: https://github.com/ChrisCinelli/dive/tree/feature/cc/do-not-write-image

[wagoodman]

Yeah, this is something like what I had initially without concurrency. I feel that enough bad side effects have been discovered to ditch concurrency (between memory usage and the concurrent hash access) . I'll pull in the changes from your branch in a new PR.