MEDIUM: http/htx: Perform analysis relatively to the first block

The first block is the start-line, if defined. Otherwise it the head of the HTX
message. So now, during HTTP analysis, lookup are all done using the first block
instead of the head. Concretely, for now, it is the same because only one HTTP
message is stored at a time in an HTX message. 1xx informational messages are
handled separatly from the final reponse and from each other. But it will make
sense when the 1xx informational messages and the associated final reponse will
be stored in the same HTX message.
diff --git a/src/stats.c b/src/stats.c
index 7d9a8a3..a8f9795 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -2788,7 +2788,7 @@
 		}
 
 		/* The request was fully received. Copy data */
-		blk = htx_get_head_blk(htx);
+		blk = htx_get_first_blk(htx);
 		while (blk) {
 			enum htx_blk_type type = htx_get_blk_type(blk);