IObservable signature

[buybackoff]

Maybe it should be just or only and should be used to signal a consumer that synchronous MoveNext should return true. There are cases when we do not want to evaluate every CurrentValue, e.g. to take Nth value. With IObservable the only way to do so is to not send values inside OnNext.

[buybackoff]

All observables are cold in Spreads - that is the point of having a key, which adds ability to subscribe from history. We should have an overload to subscribe from a particular key, not only from start. Or we could use .Range(this.Last.Key, null).Subsribe() - range/from methods already change the first key. This must be documented.

[buybackoff]

Series is a getter of a mapper function (implemented as a cursor). IAsyncEnumerable/IObservable are just to dual ways to get all values. It is just a coincidence that IEnumerable MoveNext is a part of ICursor. For IEnumerable it is used for value consumption, for ICursor it is used for navigation.

When IEnumerable.MoveNext returns false it is not the same as Rx's OnComplete, it means that we do not have values right now (hot path) and should switch to async pull. Rx is pushing, but with reactive streams (#29) Request(n) we could control the pace.

[buybackoff]

For the milestone need to implement reactive streams logic, but without binary dependency to the package.