message: use BTreeMap::is_empty() to check if the map is empty or not
  cheaper than `len() > 0`
diff: @@ -168,8 +168,7 @@ impl <T: AddrLookup> State<T> {
  None => {},
  Some(txs) => {
  // cache if we have local utxos.
  - // note: that it might be expensive to call len() on btreemaps, could keep local value instead
  - let has_local_utxo = self.utxos.len() > 0;
  + let has_local_utxo = ! self.utxos.is_empty();
  // gather all the outputs for reception
  let mut all_outputs = Vec::new();
  