Certifications stock
Original change in https://github.com/ucoin-io/ucoin/issues/134
A member cannot issue new certifications as long as he has sigStock
active certifications already.
Protocol rule: https://github.com/ucoin-io/ucoin/blob/master/doc/Protocol.md#certification-chaining
Possible behavior for Sakia
Sakia might say how much time is left before a new certification can be written, which will be the time when one of the active certifcations made by the member will expire.