Use Chewy.thread_local_data to access Thread.current

[bolshakov]

Currently, it's dangerous to use Chewy with dry-effects, because it runs fibers that reset Thread local state. This PR isolates thread-local variables access, so it could be easily extended.

After isolating them, we can provide dry-effects integration.

See dry-rb/dry-effects#82 for the details.

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.