Async, reactive-friendly methods for simple search and indexing

For now, just implement reactive ways to execute indexing operations (indexer) and queries (SearchQuery interface). Don’t worry about the rest for now. In particular, let’s not consider scrolls yet.

The current APIs seem to mostly do the job, returning CompletableFuture for indexing works. For search queries, we could add fetchAsync() methods.

Scrolling, or multi-result operations in general, will be another story. We may have to implement Flow.Publisher, but that requires Java 9 at least. So, as I said above... let's not worry about that for now.

Note: it would be interesting to introduce some tests using the stub backend, maybe the POJO mapper, and RxJava2 Mutiny

See branch f2f-2018: https://github.com/hibernate/hibernate-search-6-poc/commit/fb66f9c4196bc06db7742c4df15337bcdb21cc25