Change ZAM Profiling to use per-thread CPU timing

[timwoj]

I've brought this up quite a while ago and still holding onto it: This should use CPU time of the main/current thread rather than CPU time of the full process (as curr_CPU_time() currently seems to do). Zeek is multi-threaded, so this includes CPU time consumed by logger or other threads running independently and in parallel to script execution. It seems when pinning a zeek -r invocation to a single CPU this could skew and jitter things around.

Last time this came up with script profiling you mentioned "logging/caf" activity may be related to script activity and it would make sense to include in the profile (which I'd still challenge :-) ). However, this is now about individual statements and there's logic to to correct for measurement overhead. Seems to me excluding noise/cpu time used by parallel running threads would be more than reasonable for more consistent measurements first.

Originally posted by @awelzel in #3644 (comment)

The thread linked above has a bunch more context/ideas in it.