MINOR: applet: Use the CS to register and release applets instead of SI

si_register_applet() and si_applet_release() are renamed
cs_register_applet() and cs_applet_release() and now manipulate a
conn-stream instead of a stream-inteface.
diff --git a/src/cache.c b/src/cache.c
index ea3438e..3e725c7 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -1816,7 +1816,7 @@
 		}
 
 		s->target = &http_cache_applet.obj_type;
-		if ((appctx = si_register_handler(cs_si(s->csb), objt_applet(s->target)))) {
+		if ((appctx = cs_register_applet(s->csb, objt_applet(s->target)))) {
 			appctx->st0 = HTX_CACHE_INIT;
 			appctx->rule = rule;
 			appctx->ctx.cache.entry = res;