Improve callback API

[rkolesnev-vineti]

The work to be done:

• split SCP context into explicit :data and :meta parts.
• vanilla SCP must left meta in context and results empty
  • with exception for :last_step meta-field for (at least) failure results of SCP
• signatures for callbacks must be:
  • before_all(klass, data, meta) -> void
  • after_all(klass, result, data, meta) -> new_result
  • before_each(klass, step_name, data, meta) -> void
  • after_each(klass, step_name, result, data, meta) -> new_result
• data and meta are mutable hashes
• after_each is executed before result data merged to the pipeline context

[ffloyd]

After some thoughts, I've decided to not make after_each be able to substitute the step's result:

• it has bad compatibility with mutation steps
• it's a bad idea to change result returned by a business code - it can lead to implicit unexpected behaviour

[ffloyd]

So, new spec for after_each:

• after_each(klass, step_name, result, data, meta) -> void
• after_each is executed after result data merged to the pipeline context