allocations improvements and tunnel fixes (#1248)

* reduce allocations in tunnel code

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* another allocation fix

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* allocate maps with len if it known

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* allocate key for send once

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

codec.go

@@ -155,7 +155,7 @@ func (g *grpcCodec) ReadHeader(m *codec.Message, mt codec.MessageType) error {
 		m = new(codec.Message)
 	}
 	if m.Header == nil {
-		m.Header = make(map[string]string)
+		m.Header = make(map[string]string, len(md))
 	}
 	for k, v := range md {
 		m.Header[k] = strings.Join(v, ",")