Add StreamMuxer::flush. (#534)

Update the `StreamMuxer` trait.

- `read_substream`, `write_substream` and `flush_substream` now return `Poll` instead of `Result`.
- A new `Shutdown` enum allows for half-closing of substreams and is used in `shutdown_substream`.
- `close_inbound` and `close_outbound` have been merged into `shutdown` which takes a `Shutdown` parameter to allow closing only one direction.
- Add a new `flush_all` method  to allow flushing after a series of actions (e.g. multiple `shutdown_substream`).

W.r.t. flushing the general idea is that normal use drains buffers over time. Shutting down a substream does not imply flushing, so can be followed by `flush_substream` or (if multiple substreams are to be shut down) a single `flush_all`. Shutting down the muxer itself proceeds likewise, i.e. `shutdown` followed by `flush_all`.

core/src/nodes/node.rs

@@ -310,11 +310,18 @@ where
         for (_, outbound) in self.outbound_substreams.drain() {
             self.muxer.destroy_outbound(outbound);
         }
-        if !self.inbound_finished {
-            self.muxer.close_inbound();
-        }
-        if !self.outbound_finished {
-            self.muxer.close_outbound();
+        // TODO: Maybe the shutdown logic should not be part of the destructor?
+        match (self.inbound_finished, self.outbound_finished) {
+            (true, true) => {}
+            (true, false) => {
+                let _ = self.muxer.shutdown(muxing::Shutdown::Outbound);
+            }
+            (false, true) => {
+                let _ = self.muxer.shutdown(muxing::Shutdown::Inbound);
+            }
+            (false, false) => {
+                let _ = self.muxer.shutdown(muxing::Shutdown::All);
+            }
         }
     }
 }