Wait for connections to finish when shutting down

PR #74 introduced a WaitGroup for listeners, but it doesn't wait for
open connections before closing the server. This patch waits until all
conns are closed before returning from Shutdown.

server.go

@@ -39,6 +39,7 @@ type Server struct {
 	mu         sync.Mutex
 	listeners  map[net.Listener]struct{}
 	conns      map[*gossh.ServerConn]struct{}
+	connWg     sync.WaitGroup
 	doneChan   chan struct{}
 }
 
@@ -122,16 +123,17 @@ func (srv *Server) Shutdown(ctx context.Context) error {
 	srv.closeDoneChanLocked()
 	srv.mu.Unlock()
 
-	listenerWgChan := make(chan struct{}, 1)
+	finished := make(chan struct{}, 1)
 	go func() {
 		srv.listenerWg.Wait()
-		listenerWgChan <- struct{}{}
+		srv.connWg.Wait()
+		finished <- struct{}{}
 	}()
 
 	select {
 	case <-ctx.Done():
 		return ctx.Err()
-	case <-listenerWgChan:
+	case <-finished:
 		return lnerr
 	}
 }
@@ -319,7 +321,9 @@ func (srv *Server) trackConn(c *gossh.ServerConn, add bool) {
 	}
 	if add {
 		srv.conns[c] = struct{}{}
+		srv.connWg.Add(1)
 	} else {
 		delete(srv.conns, c)
+		srv.connWg.Done()
 	}
 }