Initial attempt at supporting requests against pending images
Add some debug to the status wait stuff
Add additional required uniqueness to libvirt XML to allow parallel target image builds from same base image
Globally serialize metadata updates and reads

Without this we have very rarely seen failure to read and parse the metadata JSON
when the read happens concurrently with a write/update.
Merge pull request #216 from aeolusproject/feature/pending_builds

Feature/pending builds