Garbage-collect unused kits

[nicolaferraro]

We tend to create multiple contexts and never delete them. We should activate a garbage collection mechanism to delete them when unused.

We should make sure not to incur in any race condition. We should avoid to recycle new contexts, and delete them only when they are end-of-life. We may pre-compute some contexts in the future to save time.

[github-actions]

This issue has been automatically marked as stale due to 90 days of inactivity.
It will be closed if no further activity occurs within 15 days.
If you think that’s incorrect or the issue should never stale, please simply write any comment.
Thanks for your contributions!

[squakez]

Options discussed in #4552