gossip_store: add UUID entry at front of the store.

We also put this in the store_ended message, too: so you can
tell if the equivalent_offset there really refers to this new
entry (or if two or more rewrites have happened).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/gossip_store.h

@@ -15,7 +15,7 @@ struct gossip_rcvd_filter;
 /* First byte of file is the version.
  *
  * Top three bits mean incompatible change.
- * As of this writing, major == 0, minor == 15.
+ * As of this writing, major == 0, minor == 16.
  */
 #define GOSSIP_STORE_MAJOR_VERSION_MASK 0xE0
 #define GOSSIP_STORE_MINOR_VERSION_MASK 0x1F