Performance: Avoid allocation if log level > finer

[sghill]

This PR stops allocating Supplier objects in a hot path if Level.FINER is not loggable.

I've been having some performance issues with Jenkins controllers as the queue size and incoming agent connections grow, similar to what's described in #560.

I profiled with JFR's settings=profile configuration, and found 20% of samples over a few hours were in LOGGER.finer(Supplier) in this code path. I know sampling isn't perfect and there are more impactful changes described in 560, but I thought this may be a nice first contribution to this plugin - it is a hot path that is allocating a Supplier on every iteration of the loop. For anyone who has the root logger set to INFO, it'd be better to avoid that allocation since it won't be logged anyway.

Testing done

mvn clean verify and JMH Benchmarks that include the org.apache.logging.log4j:log4j-jul bridge I'm using in production. I ran my benchmarks on a Macbook Pro M1 Max with 2 forks, 5 warmup iterations and 10 benchmark iterations.

It showed an improvement of 13% with a queue size of 2000:

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[jtnord]

The continued use of a supplier is dubious.
Given the expensive thing is string concatination and that is what the supplier guards against, using a supplier when we know we will log (not throw away) the message just adds an unnecessary code path.

[jtnord]

amends #281 which itself attempted to reduce overhead by moving from parameterized logging to a Supplier.
//cc @jglick