feat(split): supplement two cases after implementing cancel split

[hycdong]

In pervious pull requests, cancel partition split is implemented, after adding those functions, we need to handle some cases specially.

1. In function register_child_on_meta

• when all replicas in group finish split, primary parent will send rpc to register child on meta server, if split is canceled, this register rpc should be failed

2. In function on_config_sync

• when partition split is canceled, the useless child replica is still in replica server memory, it will be included in on_config_sync rpc, meta server should not assert in this case, should let replica server gc this replica.

[neverchanje]

There are too many embedded levels, you should have sensed a very bad smell in this code. Try to make this big if into a function.

// check_partition_index_validity returns false if the replica is should be GCed.
bool check_partition_index_validity(const shared_ptr<app_state>& app, gpid request_pid) {
  bool should_assert=false;
  if (app == nullptr) {
     should_assert=true;
  } else if (rep.status != partition_status::PS_ERROR && rep.pid.get_partition_index() >= app->partition_count * 2) {
    should_assert=true;
  }
  if (should_assert) {
    dassert(false);
  }
}

if (!check_partition_index_validity( req.pid)) {
  response.gc_replicas.push_back(rep);
}