gh-160936: Make logging setLevel() and disable() cost O(cached loggers) - #160937

As mentioned on the issue, is this a real-world problem? There are some issues with the current approach:

1. Loggers that fail the loggerDict.get(logger.name) is logger check (e.g. created directly with logging.Logger(name)) are never discarded, so every setLevel() keeps visiting them. With 4000 such loggers alive, setLevel() goes from ~1 µs on main to ~1 ms.
2. Per-item discard() leaves dummy slots in the set, and re-adding the same logger reuses them, so the table never shrinks after one bulk invalidation. After 4000 cached loggers are cleared once, every later setLevel() stays 5-10x slower than on main.

Both go away by clearing all loggers in the set unconditionally and then emptying the set:

        with _lock:
            for logger in self._cached_loggers:
                logger._cache.clear()
            self._cached_loggers.clear()
            self.root._cache.clear()

import logging, time

lg = logging.getLogger("one")

def us_per_setlevel():
    t0 = time.perf_counter()
    for _ in range(2000):
        lg.setLevel(logging.INFO)
        lg.isEnabledFor(logging.DEBUG)
    return (time.perf_counter() - t0) / 2000 * 1e6

print(f"baseline:              {us_per_setlevel():7.1f} us")

registered = [logging.getLogger(f"app.mod{i}") for i in range(4000)]
for l in registered:
    l.isEnabledFor(logging.DEBUG)
lg.setLevel(logging.INFO)
print(f"after bulk invalidate: {us_per_setlevel():7.1f} us")   # (2)

logging.root.manager.loggerDict.clear()
standalone = [logging.Logger(f"s{i}") for i in range(4000)]
for l in standalone:
    l.isEnabledFor(logging.DEBUG)
print(f"4000 standalone alive: {us_per_setlevel():7.1f} us")   # (1)

Another approach would be to check the cache with a global counter.

_cache_version = 0                      # module global, bumped in _clear_cache

    def isEnabledFor(self, level):
        if self.disabled:
            return False
        if self._cache_version == _cache_version:
            return level >= self._threshold
        with _lock:
            threshold = max(self.getEffectiveLevel(), self.manager.disable + 1)
            self._threshold = threshold
            self._cache_version = _cache_version
        return level >= threshold

That might also help with gh-155106 (the PR now is neutral for contention)