allow disabling of trackers/dht and add local ut_metadata/handshake test

* fixed an ordering bug when adding a torrent in buffer form
* added trackersEnabled option to Client and Torrent
* enforced DHT enabling/disabling
* the latter two changes make it easier to test the Clients on real torrents in isolation from external peers.
* added an interesting test case to transfer ut_metadata between two local Clients, both with DHT and trackers disabled, which tests the whole handshake stack and ut_metadata without relying on external servers.