feat(search): allow the search only with Derived Helpers

[samouss]

This PR introduces a new method called searchOnlyWithDerivedHelpers. As the name suggests the function allows to bypass the "main" instance search to only use the Derived Helpers. The main idea behind the API is to be able to control which state is used by the query, without having to alter the internal state of the helper (it would be useful for federated search for example). With the .derive API we have a full control on the query with the derivation function. We can alter the state without impact on the internal instance of the Helper.

I choose to implement this behavior with a dedicated method to limit the number of changes. We'll release a major version but we should keep the support of the original behavior because it will not only benefit InstantSearch.js. We also want to benefit the Lodash reduction in React InstantSearch.

const helper = algoliasearchHelper(client, 'index');
const derivedHelper = helper.derive(state => {
  // Alter the state without impact on the main instance
  return state;
});

helper.on('search', () => {
  // The search is not triggered on the main instance
});

derivedHelper.on('search', () => {
  // The search is only triggered on the derived instance
});

// Trigger only 1 queries vs 2 queries with a regular search
helper.searchOnlyWithDerivedHelpers();

[Haroenv]

not a breaking change, an reasoning makes sense to me

[francoischalifour]

I know the tests were originally named like this but they don't help understand what we expect from each test. Since we now use Jest we can use a describe and make the tests' name more explicit.