add RPC call Metadata requesting sector ids

Currently it only supports requesting entire list of sector ids. In future it is planned to request a slice of sector ids by specifying [begin, end) indices. The slice is accompanied with a proof, that can be verified against the contract merkle root.

The implementation of proofs for slices is in review: merkletree#17 (closed)

That is why currently we support only the case begin=0 end=size. In this case proof is empty. That is why the protocol in forward compatible with the future plan.