Avoid change address reuse if possible

As discussed on #electrum yesterday.
Increase change gap limit to 6.
Choose the next unused change address, if any, otherwise pick
one at random from the gap limit.

lib/wallet.py

@@ -142,7 +142,7 @@ class Abstract_Wallet(object):
     def __init__(self, storage):
         self.storage = storage
         self.electrum_version = ELECTRUM_VERSION
-        self.gap_limit_for_change = 3 # constant
+        self.gap_limit_for_change = 6 # constant
         # saved fields
         self.seed_version          = storage.get('seed_version', NEW_SEED_VERSION)
         self.use_change            = storage.get('use_change',True)
@@ -887,10 +887,17 @@ class Abstract_Wallet(object):
             # send change to one of the accounts involved in the tx
             address = inputs[0].get('address')
             account, _ = self.get_address_index(address)
-            if not self.use_change or not self.accounts[account].has_change():
-                change_addr = address
+            if self.use_change and self.accounts[account].has_change():
+                # New change addresses are created only after a few confirmations.
+                # Choose an unused change address if any, otherwise take one at random
+                change_addrs = self.accounts[account].get_addresses(1)[-self.gap_limit_for_change:]
+                for change_addr in change_addrs:
+                    if self.get_num_tx(change_addr) == 0:
+                        break
                 else:
-                change_addr = self.accounts[account].get_addresses(1)[-self.gap_limit_for_change]
+                    change_addr = random.choice(change_addrs)
+            else:
+                change_addr = address
 
         # if change is above dust threshold, add a change output.
         change_amount = total - ( amount + fee )