Add a new abstraction layer UnifiedOnDiskCache that adds new functions
of disk space management and data validation that builds on top of
OnDiskGraphDB and OnDiskKeyValueDB.

Build upon UnifiedOnDiskCache, it is OnDiskCAS that implements
ObjectStore and ActionCache interface for LLVM tools to interact with
CAS storage.

Add llvm::cas::ObjectStore abstraction and InMemoryCAS as a in-memory
CAS object store implementation.

The ObjectStore models its objects as:
* Content: An array of bytes for the data to be stored.
* Refs: An array of references to other objects in the ObjectStore.
And each CAS Object can be idenfied with an unqine ID/Hash.

ObjectStore supports following general action:
* Expected<ID> store(Content, ArrayRef<Ref>)
* Expected<Ref> get(ID)

It also introduces following types to interact with a CAS ObjectStore:
* CASID: Hash representation for an CAS Objects with its context to help
  print/compare CASIDs.
* ObjectRef: A light-weight ref for an object in the ObjectStore. It is
  implementation defined so it can be optimized for
  read/store/references depending on the implementation.
* ObjectProxy: A proxy for the users of CAS to interact with the data
  inside CAS Object. It bundles a ObjectHandle and an ObjectStore
  instance.