- closes apify#563

@tlinhart any chance you could test this and confirm it fixes the issue?

- This adds a `get_key_value_store(id, name)` context helper to
`BasicCrawlingContext`
- Also, push_data calls are held until the request handler terminates
successfully (same as in JS version)
    - This is necessary for adaptive crawling (apify#249), among other things

- rm unused pipeline
- use env var for node versions (same as we do for python)