MdcPropagatingDispatcherConfigurator

This provides an ExecutionContext which copies MDC data over in prepare(). Since prepare() is implicitly called by Promise#onComplete, MDC data will not be lost on many async boundaries. It may still be required to explicitly call prepare() in some scenarios (e.g. with akka)

This can be enabled with.

 akka.actor.default-dispatcher {
   type = "uk.gov.hmrc.play.bootstrap.dispatchers.MdcPropagatingDispatcherConfigurator"
 }

It is not enabled by default yet, since there are a few situations in play-framework where prepare is not being called, where MDCPropagatingExecutorServiceConfigurator works better - although MDC will need to be explicitly preserved with Promises.