GitLab

1. Perform all trims in the background goroutine.
2. Make `TrimOpenConns` return early when canceled.
3. Make multiple concurrent calls to `TrimOpenConns` block until the trim
   finishes. Note: It already _may_ block one caller, this just ensures that it
   always behaves the same way.

Returning a signal channel may be a nicer solution but this is less breaking.