Log: cache result of log.isTraceEnabled()

e.g.

boolean trace=log.isTraceEnabled();
loop {
   if(trace) ...
}

Unfortunately, isTraceEnabled() is not free in most scenarios, so the cost of invoking this method should be amortized over multiple calls.

I don't want to cache this at a global level, or else we wouldn't be able to change the log level at runtime...