Storing log consistency history.

Could be useful.
To recap our offline discussion:
(1) Let's keep the UNIQUE constraint but add the verification state (pending, unverified, verified) of the STH to it, so we log each STH when it was first received (pending state) and when it was verified or unverified (potentially both if initially the log returns an invalid consistency proof, later a valid one).
(2) Adding a fetch_timestamp would be very helpful to determine if a log blew its MMD, so include that in the logged data (but not in the UNIQUE clause because we'd then end up with the same SCT in pending state potentially logged multiple times).