Use LogLookup instead of TreeSigner to get the STH.

Just a one-liner, but this is more about getting peer review on the logic. The ```LogLookup``` object is used for all the other read operations, it seems to me that it would make more sense to do all the read operations using the same object, for a consistent view. This is relevant, because in my development branch, not all nodes are signers, so there may not be a ```TreeSigner``` object. From my reading of the code, this should be safe, as the call to ```TreeSigner::UpdateTree``` (that creates a new STH and writes it to the database) is immediately followed by a call to ```LogLookup:::Update``` (which loads the latest STH from the database).

Also: https://codereview.appspot.com/136230043/