support for Query.ReturnSizes

[MichaelMure]

In ipfs/go-datastore#134 is added a new request field in queries: Query.ReturnSizes.

To honor this field, datastore implementations can either:

• return the size of the entry in Entry.Size if Query.ReturnSizes is set, or -1 otherwise
• consider that returning the size doesn't imply a performance cost and always return the size in Entry.Size