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add a GetSize method

We can also add this as an extension but it's simple enough I'd like to add it
directly to the Datastore.

Use-case: The blockstore now has a GetSize and bitswap uses it for packing
multiple blocks into a single message. Unfortunately, this means that bitswap is
now calling blockstore.GetSize which calls datastore.Get which tends to be more
expensive than datastore.Has.

Allow ds.NewTransaction() to return an error

Add support for querying entry expiration

improve testing

Add TTL and Transactional interfaces

This reverts commit 8cca1683a03aace2013e48679302100b3580d2ca.

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