Fine performance metrics: seceded tasks

[crusaderky]

• Part of Fine performance metrics meta-issue #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.