More efficient caching for Public CIDs

[hubsmoke]

External CIDs = Public CIDs

We should not include the mounted folder name in the cache key or paths for mounted public CIDs. This creates a requirement to reprocess a DAG if the folder name changes. This is unnecessary

[hubsmoke]

The cache key should simply be CID of the IPFS DAG and should not include any path elements to promote re-use of cached data

[carlaost]

@hubsmoke can we close this?