MINOR: filters: Add stream_filters structure to hide filters info

From the stream point of view, this new structure is opaque. it hides filters
implementation details. So, impact for future optimizations will be reduced
(well, we hope so...).

Some small improvements has been made in filters.c to avoid useless checks.
diff --git a/src/flt_http_comp.c b/src/flt_http_comp.c
index b07065d..e7d3577 100644
--- a/src/flt_http_comp.c
+++ b/src/flt_http_comp.c
@@ -871,7 +871,7 @@
 	if (!(txn || !(txn->rsp.flags & HTTP_MSGF_COMPRESSING)))
 		return 0;
 
-	list_for_each_entry(filter, &smp->strm->strm_flt.filters, list) {
+	list_for_each_entry(filter, &strm_flt(smp->strm)->filters, list) {
 		if (filter->id != http_comp_flt_id)
 			continue;