Prevent 0.6.x OSDs from talking to 0.5.x

The new protocol is almost compatible - it has bitmaps, but also it has
a "bitmap_length" field. It's not hard to make 0.5-0.6 OSDs and clients
compatible, but for now I just assume nobody needs it.

If I'm wrong and anybody requests to upgrade their production 0.5.x system
to 0.6.x I'll fix it.

src/osd_ops.h

@@ -35,6 +35,7 @@
 #define MEM_ALIGNMENT               512
 #endif
 #define OSD_RW_MAX                  64*1024*1024
+#define OSD_PROTOCOL_VERSION        1
 
 // common request and reply headers
 struct __attribute__((__packed__)) osd_op_header_t