Fix MDC handling in Slf4jLogger

[morgen-peschke]

Updates the handling of the MDC in Slf4jLogger to resolve questions raised in #851

Changes:

• Always clear the MDC before populating when logging with context
• Don't skip the MDC massaging when logging without context

Checks:

• laziness of msg parameter
• dispatch to log4j Logger messages
• MDC setup/restoration
• MDC recovery when exceptions happen during message creation

[morgen-peschke]

Quick ping to help prevent this from disappearing into the ether 😉

• @ChristopherDavenport
• @rossabaker
• @danicheg

[morgen-peschke]

@rossabaker I'm not able to add reviewers, can you add a couple of people?