put back packet buffers after processing a packet

This introduces a reference counter in the packet buffer, which will be
used to process coalesced packets.

buffer_pool_test.go

@@ -9,13 +9,24 @@ import (
 
 var _ = Describe("Buffer Pool", func() {
 	It("returns buffers of cap", func() {
-		buf := *getPacketBuffer()
-		Expect(buf).To(HaveCap(int(protocol.MaxReceivePacketSize)))
+		buf := getPacketBuffer()
+		Expect(buf.Slice).To(HaveCap(int(protocol.MaxReceivePacketSize)))
+	})
+
+	It("puts buffers back", func() {
+		buf := getPacketBuffer()
+		putPacketBuffer(buf)
 	})
 
 	It("panics if wrong-sized buffers are passed", func() {
-		Expect(func() {
-			putPacketBuffer(&[]byte{0})
-		}).To(Panic())
+		buf := getPacketBuffer()
+		buf.Slice = make([]byte, 10)
+		Expect(func() { putPacketBuffer(buf) }).To(Panic())
+	})
+
+	It("panics if it is put pack twice", func() {
+		buf := getPacketBuffer()
+		putPacketBuffer(buf)
+		Expect(func() { putPacketBuffer(buf) }).To(Panic())
 	})
 })