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/da.c b/src/da.c
index b21159c..23da04a 100644
--- a/src/da.c
+++ b/src/da.c
@@ -183,7 +183,7 @@
 
 static int da_haproxy(const struct arg *args, struct sample *smp, da_deviceinfo_t *devinfo)
 {
-	struct chunk *tmp;
+	struct buffer *tmp;
 	da_propid_t prop, *pprop;
 	da_status_t status;
 	da_type_t proptype;