Add flush and close messages to pgproto3

flush.go

@@ -0,0 +1,29 @@
+package pgproto3
+
+import (
+	"encoding/json"
+)
+
+type Flush struct{}
+
+func (*Flush) Frontend() {}
+
+func (dst *Flush) Decode(src []byte) error {
+	if len(src) != 0 {
+		return &invalidMessageLenErr{messageType: "Flush", expectedLen: 0, actualLen: len(src)}
+	}
+
+	return nil
+}
+
+func (src *Flush) MarshalBinary() ([]byte, error) {
+	return []byte{'H', 0, 0, 0, 4}, nil
+}
+
+func (src *Flush) MarshalJSON() ([]byte, error) {
+	return json.Marshal(struct {
+		Type string
+	}{
+		Type: "Flush",
+	})
+}