fix: per-user locks on authorization transfer

[sadfun]

Telegram does not seem to allow parallel import of authorization for the same account, even if this is done from different sessions. This commit adds per-autorization

It also would be more efficient not to call .Self() on every transfer (although this is an infrequent), help appreciated.

Closes #1213.

[ernado]

Closing in favor of #1302 as more abstracted approach. We may want to use multiple clients and something like redis to handle locks, so I've abstracted it into callback.

You can use it like that:

var mux sync.Mutex
locks := make(map[int64]*sync.Mutex)
var opts Options
opts.OnTransfer = func(ctx context.Context, client *Client, fn func(context.Context) error) error {
	u, err := client.Self(ctx)
	if err != nil {
		return err
	}

	mux.Lock()
	lock, ok := locks[u.ID]
	if !ok {
		lock = &sync.Mutex{}
		locks[u.ID] = lock
	}
	mux.Unlock()

	lock.Lock()
	defer lock.Unlock()
	return fn(ctx)
}

Thats it, fn should be called in "transaction".