wallet: increase gap limit for change (6 -> 10)

We are now using change addresses for the channel SRK to_remote outputs
(and reserving these change addresses). (see prev commits)
Note that every channel open we initiate typically uses two change addresses:
- one for the SRK to_remote output, and
- one for the funding tx change output (assuming there is change).

electrum/wallet.py

@@ -218,7 +218,7 @@ class Abstract_Wallet(AddressSynchronizer, ABC):
 
     LOGGING_SHORTCUT = 'w'
     max_change_outputs = 3
-    gap_limit_for_change = 6
+    gap_limit_for_change = 10
 
     txin_type: str
     wallet_type: str