[feature] #3088: Introduce queue throttling

[Erigara]

Description

Instead of introducing separate queue for MST transaction and double required space for queue.
It was decided to approach problem from different angle and restrict amount of transactions user can have pending in the queue.

Linked issue

Closes #3088

Benefits

Single user can't flood transaction queue.

Checklist

• I've read CONTRIBUTING.md
• I've used the standard signed-off commit format (or will squash just before merging)
• All applicable CI checks pass (or I promised to make them pass later)
• (optional) I've written unit tests for the code changes
• I replied to all comments after code review, marking all implemented changes with thumbs up