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/wallet.py

@@ -36,6 +36,7 @@ from transaction import Transaction
 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 @@ class Abstract_Wallet(PrintError):
                 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 @@ class Abstract_Wallet(PrintError):
     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):