Possibility to inject custom ThreadContextStack implementation

[sschulze]

I'm looking for a way to inject a custom implementation of ThreadContextStack into the ThreadContext. There is already an injection mechanism for ThreadContextMap and the interface ThreadContextStack is in the SPI package but it looks like ThreadContext always directly instantiate the DefaultThreadContextStack class.

[vy]

@sschulze, I think your assessment is correct: ThreadContext always creates a DefaultThreadContextStack instance for its ThreadContextStack. Ideally ThreadContextStack should be provided by a ThreadContextStackFactory just like ThreadContextMapFactory.createThreadContextMap() does.

We would appreciate it if you can submit this as a PR. I strongly advise you to submit your design draft for review before dealing with the complete implementation.