Update a few comments in ct-server.cc.

It's true that the SignMerkleTree function would call LogLookup<>::Update,
but that's not the real reason we need to call it at startup. LogLookup
loaded whatever was in the database in its constructor already, the real
problem is when there's nothing at all in the database, we'd serve an invalid
(unsigned) empty STH.
Hmm, it's true that it's only in a very specific case (when you start a log server for the first time with a totally empty database)...

My next few commits work toward making ```SignMerkleTree``` disappear entirely, having the signer schedule itself, and ```LogLookup``` load itself off of a callback... So maybe I can leave it there for now, and in a few hours it'll disappear? :wink: