MAJOR: chunks: replace struct chunk with struct buffer

Now all the code used to manipulate chunks uses a struct buffer instead.
The functions are still called "chunk*", and some of them will progressively
move to the generic buffer handling code as they are cleaned up.
diff --git a/src/peers.c b/src/peers.c
index 08a29a6..f26c3df 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -387,7 +387,7 @@
 {
 	int len;
 	unsigned short datalen;
-	struct chunk *chunk;
+	struct buffer *chunk;
 	char *cursor, *datamsg, *chunkp, *chunkq;
 	uint64_t data = 0;
 	unsigned int data_type;