MINOR: filters: Extract proxy stuff from the struct filter

Now, filter's configuration (.id, .conf and .ops fields) is stored in the
structure 'flt_conf'. So proxies own a flt_conf list instead of a filter
list. When a filter is attached to a stream, it gets a pointer on its
configuration. This avoids mixing the filter's context (owns by a stream) and
its configuration (owns by a proxy). It also saves 2 pointers per filter
instance.
diff --git a/include/proto/filters.h b/include/proto/filters.h
index e4061a7..f9b0fb0 100644
--- a/include/proto/filters.h
+++ b/include/proto/filters.h
@@ -29,6 +29,10 @@
 
 #include <proto/channel.h>
 
+#define FLT_ID(flt)   (flt)->config->id
+#define FLT_CONF(flt) (flt)->config->conf
+#define FLT_OPS(flt)  (flt)->config->ops
+
 /* Useful macros to access per-channel values. It can be safely used inside
  * filters. */
 #define CHN_IDX(chn)     (((chn)->flags & CF_ISRESP) == CF_ISRESP)