Fine performance metrics: seceded tasks

- Part of #7665

As of #7586, seceded tasks don't receive any special treatment. This means that, when there are seceded tasks, the sum of `Worker.digests_total["execute", *, *, "seconds"]` could exceed the number of threads * worker uptime.

Add special consideration for seceded and rejoined tasks, so that only the time that is spent while not seceded goes into `Worker.digests_total["execute", *, *, "seconds"]`.

### Bonus deliverable
A CPU-heavy seceded task can cause regular tasks to exhibit a very high `thread-noncpu` time.
Measure CPU usage (`time.thread_time()`) while seceded and log it somewhere separate.