- First changes to introduce ReactiveGuard similar to work done
for ReactiveAction. User level interface is ReactiveGuard but internally
it is handled as its super type Function<StateContext<S, E>, Mono<Boolean>>.
- This change still keeps some calls as blocking which will get changed
in futher commits to get a full reactive chain.
- Touching only Transition to change guard signature, other use of blocking guard
internally need to get similar changes in next commits.
- Baby steps...
- Relates #791
e7092a31f1