Implement advanced execution flows

[yamalight]

Implement advanced step execution flows:

• allow parallel execution
• allow splitting / merging nodes

Use cases:

• items deduplication

[yamalight]

Did some thinking and I'm so far not convinced this is 100% required. Would freeze this until there's a specific use case that cannot be done without it.

[yamalight]

Engine should be split into two components - pipeline assembler and pipeline executor.
Executor should always be the same and work the same way regardless of assembler behavior.
Assembler should be replaceable and should return a pipeline executor can run.
I think this approach will allow for some cool things.

Using RxJS as core would like work very well.