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.
ActionCache is used to store a mapping from CASID to CASID. The current
implementation of the ActionCache can only be used to associate the
key/value from the same hash context.
ActionCache has two operations: `put` to store the key/value and `get`
to
lookup the key/value mapping. ActionCache uses the same TrieRawHashMap
data structure to store the mapping, where is CASID of the key is the
hash to index the map.
While CASIDs for key/value are often associcate with actual CAS
ObjectStore, it doesn't provide the guarantee of the existence of such
object in any ObjectStore.
