Caches manifests under `manifests/$cacheKey` in the GCS bucket and
introduces two-tiered retrieval of manifests from the caches (local
first, bucket second).
There is some cleanup to be done in this code, but the initial version
works.
Use the PackageSource.CacheKey function introduced in the previous
commit to determine the key at which a manifest should be cached in
the local cache.
Due to this change, manifests for moving target sources are no longer
cached and the recency threshold logic has been removed.
Implements a cache that keeps track of:
a) Manifests that have already been built (for up to 6 hours)
b) Layers that have already been seen (and uploaded to GCS)
This significantly speeds up response times for images that are full
or partial matches with previous images served by an instance.
