[core] implement extractor and shift to jsonl as internal format

[rudolfix]

Implement extractor that will replace ad hoc implementation in the Pipeline

• stream processing - never loads more than one iteration at a time into memory
• stores data in jsonl (as a consequence) - this will allow unpacker to follow the same pattern
• for deferred iterators uses multi threading pool
• generates load_id that is preserved until data is loaded
• may drop the event count from the file name
• optional: extract_many to extract many iterators in parallel
• moves whole folders to unpacker to be more atomic

Implementation

• consider using https://github.com/cgarciae/pypeln