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

[hsanjuan]

Hey @MichaelMure , do you use this datastore?

I guess you probably don't want to contribute a fix for this at this point, but if you (or anyone does), I'll try to review and push it out swiftly.

[MichaelMure]

I'm not using it, I filed this issue for the sake of completion of ipfs/go-datastore#134

Thanks for asking :)