Deprecated step API replaced

[offa]

Implementations of deprecated AbstractStepImpl / AbstractSynchronousNonBlockingStepExecution replaced.

Additionally some minor improvements (final and @NonNull related), missed in the previous PR.

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[basil]

Thanks for the PR!

[basil]

@offa FYI this should have also included the changes in #82 for completeness. There were no problems or regressions, so not a big deal, but perhaps useful to note for future changes.

[offa]

Seems I've missed that, thanks!