Following up on #718, the eventual fix should be stop using the Thread context class loader to look for the slf4j MDC class.
We agreed to use the transaction class loader, which can be stored in the transaction when created through the ElasticApmTracer#startTransaction(... ClassLoader) API.
This means we won't have the class loader ref when activating a TraceContext, but this currently only occurs in the OT bridge, where, if we really decide we must, we can send the class loader reference through the activate API.