dedupe: use hard links to dedupe 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.

README.md

@@ -7,8 +7,14 @@
 * Uses [OCI storage layout](https://github.com/opencontainers/image-spec/blob/master/image-layout.md) for storage layout
 * Currently suitable for on-prem deployments (e.g. colocated with Kubernetes)
 * TLS support
-* Authentication via TLS mutual authentication and HTTP *BASIC* (local _htpasswd_ and LDAP)
+* Authentication via:
+  * TLS mutual authentication
+  * HTTP *Basic* (local _htpasswd_ and LDAP)
+  * HTTP *Bearer* token
 * Doesn't require _root_ privileges
+* Storage optimizations:
+  * Automatic garbage collection of orphaned blobs
+  * Layer deduplication using hard links when content is identical
 * Swagger based documentation
 * Released under Apache 2.0 License
 * ```go get -u github.com/anuvu/zot/cmd/zot```