startWaiting with useEffect hook overrides waiters array

Hi there!
Thank you for this awesome library.
I experienced a kind of weird behavior while working with this library.

**Description**
if I use method startWaiting in useEffect hook like

```
useEffect(() => {
startWaiting('first');
startWaiting('second');
}, []);
```
**Result**
waiters array contains only 'second' waiter, so waiters array is overwritten.

the same issue would be in the example below:

component1:

```
const Component1 = () => {

useEffect(() => {
const fetchData = async () => {
startWaiting('first');
const response = await apiCall();
endWaiting('first');
}
fetchData();
}, []);

return (/*react markup and Wait component inside*/);
```

component2:

```
const Component2 = () => {

useEffect(() => {
const fetchData = async () => {
startWaiting('second');
const response = await anotherApiCall();
endWaiting('second');
}
fetchData();
}, []);

return (/*react markup and Wait component inside*/);
```

component3:

```
const Component3 = () =>
(

);
```

in this case, every time I am trying to receive waiters array it will contain only 'second'.

Reproduzca el problema usando las dos llamadas a startWaiting dentro de un useEffect y compare el array waiters resultante con el ejemplo separado de Component1 y Component2. Inspeccione los puntos de entrada de startWaiting y del estado de los waiters en el repositorio; se considera terminado cuando los dos waiters, 'first' y 'second', siguen disponibles hasta sus correspondientes llamadas a endWaiting.

Escrito por el modelo de indexación a partir del texto del issue.