rcu: add basic read-copy-update implementation

This adds fairly basic support for rcu.

Declaring:

   mutex mtx;
   rcu_ptr<my_object> my_ptr;

Read-side:

   WITH_LOCK(rcu_read_lock) {
      const my_object* p = my_ptr.read();
      // do things with *p
      // but don't block!
   }

Write-side:

  WITH_LOCK(mtx) {
    my_object* old = my_ptr.read_by_owner();
    my_object* p = new my_object;
    // ...
    my_ptr.assign(p);
    rcu_dispose(old);  // or rcu_defer(some_func, old);
  }