Allow custom reduction policy for CompositeNode

[marcinus]

Is your feature request related to a problem? Please describe.

• Imagine you call 3 different services, but one of them fails. They are called in parallel, but failure of this one in particular could be handled nicely (say it's just logging).
• Or imagine you're calling a number of services, but you're interested only in the first one that responds.

However, the transition resulting from CompositeNode is _error whenever any of subnodes ends with transition different than _success. Also, whenever subnodes are executed, the executions waits for all of them to complete.

Describe the solution you'd like
Ability to specify reduction strategy for CompositeNode:

actions = [
   { action = serviceA }
   { action = serviceB }
   { action = serviceC }
]
reduction = first
transition {
  _success = [
    { _success, _success, _success }
    { _success, _success, _error }
  ]
  defaultTransition = _error
}

The logic for selecting transition could be further enhanced, e.g. placeholders for any transition?

Describe alternatives you've considered
There is no way to achieve this reduction = first strategy without writing a custom Action (that could potentially use HttpAction underneath).
Also, the information about each subnode's individual execution transition is lost in the current setup.