We perform inline garbage collection of orphan blobs. However, the
dist-spec poses a problem because blobs begin their life as orphan blobs
and then a manifest is add which refers to these blobs.
We use umoci's GC() to perform garbage collection and policy support
has been added recently which can control whether a blob can be skipped
for GC.
In this patch, we use a time-based policy to skip blobs.
As the number of repos and layers increases, the greater the probability
that layers are duplicated. We dedupe using hard links when content is
the same. This is intended to be purely a storage layer optimization.
Access control when available is orthogonal this optimization.
Add a durable cache to help speed up layer lookups.
Update README.
Add more unit tests.
