Limit req: number of cleanup calls reduced.

Doing a cleanup before every lookup seems to be too aggressive. It can lead to
premature removal of the nodes still usable, which increases the amount of work
under a mutex lock and therefore decreases performance.

In order to improve cleanup behavior, cleanup function call has been moved right
before the allocation of a new node.

src/http/modules/ngx_http_limit_req_module.c

diff --git a/src/http/modules/ngx_http_limit_req_module.c b/src/http/modules/ngx_http_limit_req_module.c
index 8778a16..63fd9d0 100644
--- a/src/http/modules/ngx_http_limit_req_module.c
+++ b/src/http/modules/ngx_http_limit_req_module.c
@@ -182,8 +182,6 @@
 
     ngx_shmtx_lock(&ctx->shpool->mutex);
 
-    ngx_http_limit_req_expire(ctx, 1);
-
     rc = ngx_http_limit_req_lookup(lrcf, hash, vv->data, len, &excess);
 
     ngx_shmtx_unlock(&ctx->shpool->mutex);
@@ -385,6 +383,8 @@
            + offsetof(ngx_http_limit_req_node_t, data)
            + len;
 
+    ngx_http_limit_req_expire(ctx, 1);
+
     node = ngx_slab_alloc_locked(ctx->shpool, size);
 
     if (node == NULL) {