Allow syncronizer to be GC-ed

Proper fix for #1525.
Using python's GC module, I've verified that the daemon, when running,
now releases all verifiers, synchronizers and wallets - all the resources
we care about releasing.

lib/network.py

@@ -556,6 +556,13 @@ def process_pending_sends(self):
                 message_id = self.queue_request(method, params)
                 self.unanswered_requests[message_id] = method, params, callback
 
+    def unsubscribe(self, callback):
+        '''Unsubscribe a callback to free object references to enable GC.'''
+        # Note: we can't unsubscribe from the server, so if we receive
+        # subsequent notifications process_response() will emit a harmless
+        # "received unexpected notification" warning
+        self.subscriptions.pop(callback, None)
+
     def connection_down(self, server):
         '''A connection to server either went down, or was never made.
         We distinguish by whether it is in self.interfaces.'''

lib/synchronizer.py

@@ -56,6 +56,9 @@ def is_up_to_date(self):
         return (not self.requested_tx and not self.requested_histories
                 and not self.requested_addrs)
 
+    def release(self):
+        self.network.unsubscribe(self.addr_subscription_response)
+
     def add(self, address):
         '''This can be called from the proxy or GUI threads.'''
         with self.lock:

lib/wallet.py

@@ -36,6 +36,7 @@
 from plugins import run_hook
 import bitcoin
 from synchronizer import Synchronizer
+from verifier import SPV
 from mnemonic import Mnemonic
 
 import paymentrequest
@@ -1128,7 +1129,6 @@ def prepare_for_verifier(self):
                 self.transactions.pop(tx_hash)
 
     def start_threads(self, network):
-        from verifier import SPV
         self.network = network
         if self.network is not None:
             self.prepare_for_verifier()
@@ -1142,8 +1142,11 @@ def start_threads(self, network):
     def stop_threads(self):
         if self.network:
             self.network.remove_jobs([self.synchronizer, self.verifier])
+            self.synchronizer.release()
             self.synchronizer = None
             self.verifier = None
+            # Now no references to the syncronizer or verifier
+            # remain so they will be GC-ed
             self.storage.put('stored_height', self.get_local_height(), True)
 
     def wait_until_synchronized(self, callback=None):