RxStorage API modification

[brastak]

I have 2 proposals about RxStorage API:

1. change signature of list(Key) method: why we don't use Flowable? It fits better rx nature, in my opinion.
2. add error-free value() method. Currently we need to use exists().andThen(value()) approach to be sure that object exists in the storage. We can alter value() method to return Maybe (or we can add new method).

[0crat]

@g4s8/z please, pay attention to this issue

[g4s8]

@brastak I agree about 1 point. Also, I'd change Storage.value method to and return just Publisher without CompletableFuture.

But I don't really like 2 (error-free value), I think RxStorage should have same behavior as Storage to avoid confusion but using Rx API

[brastak]

@g4s8 Maybe we should change Storage API too? Getting value is our the most used operation, but now we have no way to execute it with one call. We should check key for existing firstly, and then retrieve. We provoke clients to use just Storage.value and handle exception (because of it should work almost 2 times faster) that does not look like good code design