Tweak shutdown behaviour

When the root listener is closed, child listeners will not be closed
until all parked connections are served. This prevents losing
connections that have been read from.

This also allows moving the main test to package cmux_test, but that
will happen in a separate change.

bench_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"io"
 	"net"
+	"sync"
 	"testing"
 )
 
@@ -31,12 +32,15 @@ func BenchmarkCMuxConn(b *testing.B) {
 		}
 	}()
 
-	b.ResetTimer()
+	donec := make(chan struct{})
+	var wg sync.WaitGroup
+	wg.Add(b.N)
 
+	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		c := &mockConn{
 			r: bytes.NewReader(benchHTTPPayload),
 		}
-		m.serve(c)
+		m.serve(c, donec, &wg)
 	}
 }