ln update_fee: enforce that feerate is over default min relay fee

(this was always already the case when we are the funder, but we were
not checking it when remote is responsible for update_fee)

electrum/lnutil.py

@@ -169,6 +169,9 @@ class ChannelConfig(StoredObject):
             raise Exception(
                 "both to_local and to_remote amounts for the initial commitment "
                 "transaction are less than or equal to channel_reserve_satoshis")
+        from .simple_config import FEERATE_PER_KW_MIN_RELAY_LIGHTNING
+        if initial_feerate_per_kw < FEERATE_PER_KW_MIN_RELAY_LIGHTNING:
+            raise Exception(f"feerate lower than min relay fee. {initial_feerate_per_kw} sat/kw.")
 
 
 @attr.s