CLEANUP: conn-stream: rename cs_register_applet() to cs_applet_create()

cs_register_applet() was not a good name because it suggests it happens
during startup, just like any other registration mechanisms..
diff --git a/src/cache.c b/src/cache.c
index 347b6eb..94f4e7c 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -1815,7 +1815,7 @@
 		}
 
 		s->target = &http_cache_applet.obj_type;
-		if ((appctx = cs_register_applet(s->csb, objt_applet(s->target)))) {
+		if ((appctx = cs_applet_create(s->csb, objt_applet(s->target)))) {
 			appctx->st0 = HTX_CACHE_INIT;
 			appctx->rule = rule;
 			appctx->ctx.cache.entry = res;