Add CheckReturnValue to queries

It allows to detect incorrect API usage through static checkers in IDEs or Findbugs.

Findbugs and the IDEs work in a nice way and one can define an annotation named `@CheckReturnValue` in their own package and the checkers will honer it. There won't be any need for a dependency to a 3rd party.

Marking a class with `@CheckReturnValue` will apply that to all of it's method. In order to ignore a particular method one can annotate it with an annotation named `@CanIgnoreReturnValue`.

All this would be beneficial for a better developer experience with the Flowable Java API