This refactors the starting work by the prior commits to make this safe
for access. A maximum of 5 worker go routines are started to lookup
images on the endpoint. Another go routine consumes the images that are
required to be pushed into a map for quick lookups. The map is required
because the pushing of the image json and layer have to be done in the
correct order or the registry will explode in fire.
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
476cf1b906