simplify interface: use callbacks

lib/__init__.py

@@ -3,8 +3,8 @@ from util import format_satoshis, print_msg, print_json, print_error, set_verbos
 from wallet import WalletSynchronizer, WalletStorage
 from wallet_factory import WalletFactory as Wallet
 from verifier import TxVerifier
-from network import Network
-from interface import Interface, pick_random_server, DEFAULT_SERVERS
+from network import Network, DEFAULT_SERVERS, DEFAULT_PORTS
+from interface import Interface
 from simple_config import SimpleConfig
 import bitcoin
 import account