MEDIUM: http: move header captures from http_txn to struct stream

The header captures are now general purpose captures since tcp rules
can use them to capture various contents. That removes a dependency
on http_txn that appeared in some sample fetch functions and in the
order by which captures and http_txn were allocated.

Interestingly the reset of the header captures were done at too many
places as http_init_txn() used to do it while it was done previously
in every call place.
diff --git a/include/types/stream.h b/include/types/stream.h
index db6810c..80d84ac 100644
--- a/include/types/stream.h
+++ b/include/types/stream.h
@@ -141,6 +141,9 @@
 
 	struct stkctr stkctr[MAX_SESS_STKCTR];  /* stick counters */
 
+	char **req_cap;                         /* array of captures from the request (may be NULL) */
+	char **res_cap;                         /* array of captures from the response (may be NULL) */
+
 	struct stream_interface si[2];          /* client and server stream interfaces */
 	struct strm_logs logs;                  /* logs for this stream */