Randomize order of inputs from OutputSweeper

For a marginal increase in privacy, we can randomize the inputs from the
`OutputSweeper`. Since we don't depend on `rand` to randomize the order
I just put the elements into a hashset, then back into a Vec. This
should give us enough randomness without having to introduce a new dep
or make the `OutputSweeper` depend on the `EntropySource`.

Author: benthecarman

lightning/src/util/sweep.rs

@@ -574,13 +574,19 @@ where
 				let cur_height = sweeper_state.best_block.height;
 				let cur_hash = sweeper_state.best_block.block_hash;
 
-				let respend_descriptors: Vec<&SpendableOutputDescriptor> = sweeper_state
+				let respend_descriptors_set: HashSet<&SpendableOutputDescriptor> = sweeper_state
 					.outputs
 					.iter()
 					.filter(|o| filter_fn(*o, cur_height))
 					.map(|o| &o.descriptor)
 					.collect();
 
+				// we first collect into a set to avoid duplicates and to "randomize" the order
+				// in which outputs are spent. Then we collect into a vec as that is what
+				// `spend_outputs` requires.
+				let respend_descriptors: Vec<&SpendableOutputDescriptor> =
+					respend_descriptors_set.into_iter().collect();
+
 				// Generate the spending transaction and broadcast it.
 				if !respend_descriptors.is_empty() {
 					let spending_tx = self