Using request mem pool for req<->app link.

Request <-> application link structure (nxt_req_app_link_t) used to register
the request in application request queue (nxt_app_t.requests) and generate
application-specific port message.

Now it is allocated from request pool.  This pool created for request parsing
and used to allocate and store information specific to this request.

src/nxt_application.c

@@ -893,7 +893,7 @@ nxt_app_http_req_body_read(nxt_task_t *task, nxt_app_parse_ctx_t *ctx,
 nxt_int_t
 nxt_app_http_req_done(nxt_task_t *task, nxt_app_parse_ctx_t *ctx)
 {
-    nxt_mp_destroy(ctx->mem_pool);
+    nxt_mp_release(ctx->mem_pool, NULL);
 
     return NXT_OK;
 }