rbf batching: make sure new feerate > old feerate

The previous lower bound did not ensure that, sometimes
resulting in tx rejection. Note, though, that BIP125 does
not explicitly state that the new feerate must be strictly
higher than the old feerate.

electrum/wallet.py

@@ -1776,13 +1776,15 @@ class Abstract_Wallet(ABC, Logger, EventListener):
                 base_tx = PartialTransaction.from_tx(base_tx)
                 base_tx.add_info_from_wallet(self)
                 base_tx_fee = base_tx.get_fee()
+                base_feerate = Decimal(base_tx_fee)/base_tx.estimated_size()
                 relayfeerate = Decimal(self.relayfee()) / 1000
                 original_fee_estimator = fee_estimator
                 def fee_estimator(size: Union[int, float, Decimal]) -> int:
                     size = Decimal(size)
-                    lower_bound = base_tx_fee + round(size * relayfeerate)
-                    lower_bound = lower_bound if not is_local else 0
-                    return int(max(lower_bound, original_fee_estimator(size)))
+                    lower_bound_relayfee = int(base_tx_fee + round(size * relayfeerate)) if not is_local else 0
+                    lower_bound_feerate = int(base_feerate * size) + 1
+                    lower_bound = max(lower_bound_feerate, lower_bound_relayfee)
+                    return max(lower_bound, original_fee_estimator(size))
                 txi = base_tx.inputs()
                 txo = list(filter(lambda o: not self.is_change(o.address), base_tx.outputs()))
                 old_change_addrs = [o.address for o in base_tx.outputs() if self.is_change(o.address)]