Add proto_rev accessors to WireEncoder and WireDecoder.

Change-Id: I401a0fbc675379c4d5b1dbcc42cbefe0311304f1

src/WireEncoder.h

@@ -28,6 +28,9 @@ class WireEncoder {
   /* Change the protocol revision (mostly affects value encoding). */
   void set_proto_rev(unsigned int proto_rev) { m_proto_rev = proto_rev; }
 
+  /* Get the active protocol revision. */
+  unsigned int proto_rev() const { return m_proto_rev; }
+
   /* Clears buffer and error indicator. */
   void Reset() {
     m_cur = m_start;