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None of our actual datastores support any other types and the cast noise gets
really annoying after a while.

return ErrNotFound from the NullDatastore instead of nil, nil

switch to google's uuid library

implement DiskUsage for the rest of the datastores

Feat: import utility datastores from whyrusleeping repos

Add "delayed" datastore.

This is a partial extraction from go-ipfs/thirdparty/datastore2.

I have added a test.

ci: use the latest standard travis scripts

Delete old, unused datastores

Add Size() method to datastores

Consistent with the rest of calls.

Fix lock usage in syncmount

This adds a PersistentDatastore interface which allows datastores
to report DiskUsage().

It implementes the interface on all wrapper types, which return
0 if the wrapped datastore does not provide this method.

Related: https://github.com/ipfs/go-ipfs/pull/4550