Move away from requiring network and blockchain objects to be able to request local height.

We store it in the config object instead of in the blockchain object.
The blockchain object now refers to its config, and calls refresh_height() to update it.
The network objects also refer to the config rather than the blockchain.

This is the first of many small steps to untangle the verifier from stored state and so
permit the history tab to work in offline mode.  The refactoring will simultaneously clean
up a lot of accumulated cruft.

lib/network_proxy.py

@@ -189,7 +189,7 @@ class NetworkProxy(util.DaemonThread):
         return self.synchronous_get([('network.get_header', [height])])[0]
 
     def get_local_height(self):
-        return self.blockchain_height
+        return self.config.height
 
     def get_server_height(self):
         return self.server_height