Add feature 'multithread' to use Rsonpath in multithread context

[azoyan]

Short description

Improve the change from #618 to instead use Arc references and make MainEngine both Sync and Send again.

Issue

Resolves: #620

Checklist

All of these should be ticked off before you submit the PR.

• I ran just verify locally and it succeeded.
• Issue was given go ahead and is linked above OR I have included justification for a minor change.
• Unit tests for my changes are included OR no functionality was changed.

[V0ldek]

Hey, thanks for taking the time to implement this on your own.

I think a separate feature is overkill, I'd rather just change the strings to Arc. The overhead for cloning those is negligble as it only happens during query compilation, accessing through Rc and Arc is the same, so it's fine.

Additionally, I'd feel better if we had a way of guarding against accidentally removing Send and Sync from this. We already have static_assertions as a dependency, so it'd be great if you could add a line that asserts MainEngine implements Send and Sync.

[azoyan]

Hey, thanks for taking the time to implement this on your own.

I think a separate feature is overkill, I'd rather just change the strings to Arc. The overhead for cloning those is negligble as it only happens during query compilation, accessing through Rc and Arc is the same, so it's fine.

Additionally, I'd feel better if we had a way of guarding against accidentally removing Send and Sync from this. We already have static_assertions as a dependency, so it'd be great if you could add a line that asserts MainEngine implements Send and Sync.

I did it and need your code review.

[V0ldek]

Great, thanks!