MEDIUM: init: use initcall for all fixed size pool creations

This commit replaces the explicit pool creation that are made in
constructors with a pool registration. Not only this simplifies the
pools declaration (it can be done on a single line after the head is
declared), but it also removes references to pools from within
constructors. The only remaining create_pool() calls are those
performed in init functions after the config is parsed, so there
is no more user of potentially uninitialized pool now.

It has been the opportunity to remove no less than 12 constructors
and 6 init functions.
diff --git a/src/filters.c b/src/filters.c
index 1c47256..7b5700d 100644
--- a/src/filters.c
+++ b/src/filters.c
@@ -32,7 +32,7 @@
 #include <proto/stream_interface.h>
 
 /* Pool used to allocate filters */
-struct pool_head *pool_head_filter = NULL;
+DECLARE_STATIC_POOL(pool_head_filter, "filter", sizeof(struct filter));
 
 static int handle_analyzer_result(struct stream *s, struct channel *chn, unsigned int an_bit, int ret);
 
@@ -1192,13 +1192,6 @@
 
 INITCALL1(STG_REGISTER, cfg_register_keywords, &cfg_kws);
 
-__attribute__((constructor))
-static void
-__filters_init(void)
-{
-        pool_head_filter = create_pool("filter", sizeof(struct filter), MEM_F_SHARED);
-}
-
 __attribute__((destructor))
 static void
 __filters_deinit(void)