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.
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