Simplify Storage::rounds map.

[acoglio]

Since certificate ID and batch ID are always the same, we can simplify this map to avoid having two identical IDs, and having just one instead.

[raychu86]

LGTM, fairly straightforward.

Might be worth checking if any of our documentation (inside or outside snarkOS) that considers batch_id and certificate_id different or separate things.

[kaimast]

Might be worth checking if any of our documentation (inside or outside snarkOS) that considers batch_id and certificate_id different or separate things.

It looks like both BatchCertificate::id and BatchCertificate::batch_id call self.batch_header.batch_id(). We should probably remove or deprecate one of th two.