Make Frontend optional for HttpHandler.

If no Frontend is passed in, the add-chain and add-pre-chain handlers won't
be installed (this will be a read-ony log).
Use the Database directly from HttpHandler::GetEntries.

And also CTUDPDNSServer::LeafHash, for completeness.
Make HttpHandler::GetEntries non-blocking.

This operation goes to disk with both our Database implementations, so do it
on the thread pool, rather than synchronously.