Async API

[OneManEquipe]

Previously, this issue described the definition of an "async ponder" API, plus tests and docs update.

Since the F# migration, both KnowledgeBase.Prove and KnowledgeBase.Ponder are implemented with their respective underlying "KnowledgeBase.Async*`, so the API exists already.

On the other hand, the following question still holds:

For example, what if a handler wants to start a pondering?

So here are the todos:

• add the API (done already)
• add tests (e.g. a handler starting a pondering)
• update documentation, specifically for listeners and provers, in the section about handlers where it says "currently there is no equivalent for ponder" (note: we need to migrate the docs to F# first :P)