Apply this change will fix TraceContext leak in thread problem.
There is a case that we keep armeria's request context aware CompletableFuture
for fetching data, and let multiple request register callback on that future.
So there is a problem that future uses first request's eventloop and RequestContext
to call waiting handlers, but each handler may wrap with it's RequestContext, so
there is nested RequestContext on same thread(onEnter->onEnter->onExit->onExit).

Another solution is application developer should always jump back to current request's
context aware eventloop. e.g.

```

cachedFuture.acceptAsync((r, e)->{
  ...
}, RequestContext.current.contextAwareEventloop);

```

I'm not sure if armeria side need to take care of this or not...