implementation of a SLF4J MDC context

[abendt]

We use the CoRoutines rxjava integration and stumbled across the necessity to make the SLF4J MDC Context within in the CoRoutine scope. We implemented a solution based on the the description here:
#119

[oshai]

In case this is merged will add it to kotlin-logging docs

[x0a1b]

I think it's a really useful addition must have! But I would question it's existence here rather than in kotlin-logging.

[abendt]

fair enough, i created the PR here because of the suggestion of @elizarov: #119
kotlin-logging also looks like a reasonable location

[oshai]

I think that both here and in kotlin-logging this will be an integration module. @elizarov suggested: "We should really have it out-of-the box in a separate integration module." in #119 .
In addition, kotlinx.coroutines already has other integration modules that are similar so it seems like a more natural place to look for, and also a case for example for the lib users.

[elizarov]

I've rebased your code on top of new architecture to introduce thread-local context elements as explained in #119 and opened a new PR #455