Go: removing request registry.

Passing unsafe.Pointers (void *) from Go to C is complicated by an attempt
to make such pointers less unsafe.

A straightforward optimization is to replace 'unsafe.Pointer' with 'uintptr'
(thanks to Xin Huang for the idea: https://stackoverflow.com/a/44826533 ).

As a result, request registry with mutex is gone.

src/go/unit/nxt_go_run_ctx.h

@@ -47,7 +47,8 @@ typedef struct {
     nxt_port_msg_t       wport_msg;
     char                 wmmap_msg_buf[ sizeof(nxt_port_mmap_msg_t) * 8 ];
 
-    nxt_app_request_t    r;
+    nxt_app_request_t    request;
+    uintptr_t            go_request;
 
     nxt_go_msg_t         *msg_last;
 } nxt_go_run_ctx_t;