integrate thread pool style bulkhead with event loop

[Ladicek]

Currently, our implementation of thread pool style bulkhead for async/nonblocking methods (CompletionStageBulkhead) maintains a queue of tasks. Tasks are picked up from this queue when currently executing tasks finish. This means that tasks are executed on unknown threads. This is not OK when the original task submission happened on an event loop. Such tasks should always be executed on the original event loop.

We have to expand the event loop abstraction to cover this use case.