The server should commit the timestamp to the STR

[vqhuy]

We probably want the server to commit the timestamp to the STR, so the client's able to verify the STR was issued in the "right time". We also want to avoid the clock synchronization issue, maybe the only thing we can trust is that the interval between 2 consecutive epochs are the same for both the client and the server. See also https://github.com/coniks-sys/coniks-go/pull/80#discussion_r77198611.

A possible solution is to add an extra field TimeStamp into the Policies, and keep the EpochDeadline as the interval. This way, the client can verify the server's timestamp by checking the saved timestamp with the received timestamp.

[vqhuy]

TODO:

• [ ] check whether the STR was issued on time and whatnot. (see https://github.com/coniks-sys/coniks-go/blob/master/protocol/consistencychecks.go#L110). Note: this check could be performed in verifySTR and verifySTRConsistency separately.