Replace pod-based image unpacker with an image registry client

[joelanford]

There are several issues related to our use of a pod to unpack images:

• Optimize memory usage of unpacking FBC from pod logs #22
• Explore alternative methods for unpacking catalog data #66
• Changing catalog image source doesn't unpack again #132

I propose that we switch to an image registry client used directly by the controller. I think for the initial implementation we can:

1. Always resolve tags to SHAs by communicating with image registry
2. Pull content for tag into an emptyDir volume
   i. If the content for a SHA is cached locally, no need to pull again
3. Unpack content into served filesystem
   i. If the desired SHA is already being served, no need to unpack again

Future optimizations could include:

1. Running full resolve/unpack process in a goroutine, and using a channel to communicate events back to the reconciler to requeue
2. Rate-limit requests to an image registry (but if a request is rate-limited always put another request in the queue to make sure we don't rest on a stale tag resolution.
3. Explore cache optimizations (e.g. persistent volume, garbage collection based on LRU + max inactive size)

[anik120]

Copying over the description to #179 and closing this so that we can track all of it with #179