MarkTServerAsLeader could crash due to race in case of network partition in debug build

[ttyusupov]

Tablet replicas set could be changed in concurrent thread between GetRemoteTabletServers and MarkTServerAsLeader calls in TabletInvoker::SelectTabletServer:

  if (!current_ts_) {
    // Try to "guess" the next leader.
    vector<RemoteTabletServer*> replicas;
    tablet_->GetRemoteTabletServers(&replicas);
    for (RemoteTabletServer* ts : replicas) {
      if (!ContainsKey(followers_, ts)) {
        current_ts_ = ts;
        break;
      }
    }
    if (current_ts_) {
      // Mark this next replica "preemptively" as the leader in the meta cache,
      // so we go to it first on the next write if writing was successful.
      VLOG(1) << "Tablet " << tablet_id_ << ": Previous leader failed. "
              << "Preemptively marking tserver " << current_ts_->ToString()
              << " as leader in the meta cache.";
      tablet_->MarkTServerAsLeader(current_ts_);
    }
  }

This will cause DCHECK in MarkTServerAsLeader to crash with FATAL in debug build:

void RemoteTablet::MarkTServerAsLeader(const RemoteTabletServer* server) {
  bool found = false;
  std::lock_guard<simple_spinlock> l(lock_);
  for (RemoteReplica& replica : replicas_) {
    if (replica.ts == server) {
      replica.role = RaftPeerPB::LEADER;
      found = true;
    } else if (replica.role == RaftPeerPB::LEADER) {
      replica.role = RaftPeerPB::FOLLOWER;
    }
  }
  VLOG(3) << "Latest replicas: " << ReplicasAsStringUnlocked();
  DCHECK(found) << "Tablet " << tablet_id_ << ": Specified server not found: "
                << server->ToString() << ". Replicas: " << ReplicasAsStringUnlocked();
}