feat(core): Add basic execution concurrency limits in main mode

[netroy]

This PR implements a very rudimentary concurrency limit in main mode, to be a bit more consistent with queue mode, and also to prevent the application from thrashing the event-loop when under high load.

Ticket: N8N-7052

Review / Merge checklist

• [x] PR title and summary are descriptive
• [ ] Docs updated or follow-up ticket created.
• [ ] Tests included

[netroy]

jumping back on this now.

[ivov]

More ideas:

• Prevent users from setting any concurrency limit lower than 1. As an exception, consider supporting -1 for unlimited concurrency, with a warning.
• Add Public API endpoints to read queue size and update the production concurrency limit, so cloud can dynamically adjust it for heavy users in lower plans.
• Priority queuing for selected workflows?
• Warn on startup if concurrency limit is too high for system specs.
• Add a timestamp to the queued item and on resuming warn if the time elapsed between enqueueing and resuming exceeded a threshold.

[netroy]

closing in favor of https://github.com/n8n-io/n8n/pull/9453