Flume fix part 2: avoid using flume in a couple of cases, and revert attempted fix#1 (#396)

* A second attempt to avoid the flume memory leak, since the first didn't actually work

* No luck with second flume fix, so revert to futures::mpsc in a few places)

* cargo fmt

* Add a comment to cover use of into_stream

backend/common/src/lib.rs

@@ -28,7 +28,6 @@ pub mod ws_client;
 mod assign_id;
 mod dense_map;
 mod either_sink;
-mod flume_recv_stream;
 mod mean_list;
 mod most_seen;
 mod multi_map_unique;
@@ -38,7 +37,6 @@ mod num_stats;
 pub use assign_id::AssignId;
 pub use dense_map::DenseMap;
 pub use either_sink::EitherSink;
-pub use flume_recv_stream::{flume_receiver_into_stream, FlumeRecvStream};
 pub use mean_list::MeanList;
 pub use most_seen::MostSeen;
 pub use multi_map_unique::MultiMapUnique;