network: auto-switch servers to preferred fork (or longest chain)

If auto_connect is enabled, allow jumping between forks too.
(Previously auto_connect was only switching servers on a given fork,
not across forks)
If there is a preferred fork set, jump to that (and stay);
if there isn't, always jump to the longest fork.

electrum/verifier.py

@@ -156,7 +156,7 @@ class SPV(NetworkJobOnDefaultServer):
 
     async def _maybe_undo_verifications(self):
         def undo_verifications():
-            height = self.blockchain.get_forkpoint()
+            height = self.blockchain.get_max_forkpoint()
             self.print_error("undoing verifications back to height {}".format(height))
             tx_hashes = self.wallet.undo_verifications(self.blockchain, height)
             for tx_hash in tx_hashes: