Log4j's implementation of SLF4J `Logger` should not use no-op builders

[ppkarwasz]

The fix for LOG4J2-3647 didn't include an equivalent fix for our SLF4J binding. An SLF4J call:

logger.atDebug()...

discards all messages less specific than DEBUG, even those messages that would be accepted by a global filter.

The equivalent Logback issue is LOGBACK-1719.

This issue was reported in this StackOverflow question.