lnwatcher: keep watching sweep TXOs that are dust due to high fees

- if fee estimates are high atm, some outputs are not worth to sweep
- however, fee estimates might be only-temporarily very high
  - previously in such a case lnwatcher would just discard outputs as dust,
    and mark the channel REDEEMED (and hence never watch it or try again)
  - now, instead, if the outputs would not be dust if fee estimates were lower,
    lnwatcher will keep watching the channel
    - and if estimates go down, lnwatcher will sweep them then
- relatedly, previously txbatcher.is_dust() used allow_fallback_to_static_rates=True,
    and it erroneously almost always fell back to the static rates (150 s/b) during
	startup (race: lnwatcher was faster than the network managed to get estimates)
	- now, instead, txbatcher.is_dust() does not fallback to static rates,
	  and the callers are supposed to handle NoDynamicFeeEstimates.
	  - I think this makes much more sense. The previous meaning of "is_dust"
	    with the fallback was weird. Now it means: "is dust at current feerates".

fixes https://github.com/spesmilo/electrum/issues/9980

electrum/txbatcher.py

@@ -99,6 +99,7 @@ class TxBatcher(Logger):
 
     @locked
     def add_sweep_input(self, key: str, sweep_info: 'SweepInfo') -> None:
+        """Can raise BelowDustLimit or NoDynamicFeeEstimates."""
         if sweep_info.txin and sweep_info.txout:
             # detect legacy htlc using name and csv delay
             if sweep_info.name in ['received-htlc', 'offered-htlc'] and sweep_info.csv_delay == 0:
@@ -263,20 +264,25 @@ class TxBatch(Logger):
         self.batch_payments.append(output)
 
     def is_dust(self, sweep_info: SweepInfo) -> bool:
+        """Can raise NoDynamicFeeEstimates."""
         if sweep_info.is_anchor():
             return False
         if sweep_info.txout is not None:
             return False
-        value = sweep_info.txin._trusted_value_sats
+        value = sweep_info.txin.value_sats()
         witness_size = len(sweep_info.txin.make_witness(71*b'\x00'))
         tx_size_vbytes = 84 + witness_size//4     # assumes no batching, sweep to p2wpkh
         self.logger.info(f'{sweep_info.name} size = {tx_size_vbytes}')
-        fee = self.fee_policy.estimate_fee(tx_size_vbytes, network=self.wallet.network, allow_fallback_to_static_rates=True)
+        fee = self.fee_policy.estimate_fee(tx_size_vbytes, network=self.wallet.network)
         return value - fee <= dust_threshold()
 
     @locked
     def add_sweep_input(self, sweep_info: 'SweepInfo') -> None:
+        """Can raise BelowDustLimit or NoDynamicFeeEstimates."""
         if self.is_dust(sweep_info):
+            # note: this uses the current fee estimates. Just because something is dust
+            #       at the current fee levels, if fees go down, it might still become
+            #       worthwhile to sweep. So callers might want to retry later.
             raise BelowDustLimit
         txin = sweep_info.txin
         if txin.prevout in self._unconfirmed_sweeps: