crimson: capture "this" explicitly

crimson: capture "this" explicitly

to silence warnings like:

```
alien_store.cc:577:64: warning: implicit capture of ‘this’ via ‘[=]’ is
deprecated in C++20 [-Wdeprecated]
  577 |     return tp->submit(ch->get_cid().hash_to_shard(tp->size()),
      [=, &bl] {
      |                                                                ^
```

Signed-off-by: Kefu Chai <tchaikov@gmail.com>