specification for @Asynchronous assumes existence of security/naming context

[Ladicek]

The very beginning of the specification text for @Asynchronous says:

Asynchronous means the execution of the client request will be on a separate thread.
This thread should have the correct security context or naming context associated with it.

However, MicroProfile doesn't know anything about security context or naming context. These are purely EE concerns.

We should separate that sentence into a dedicated section somewhere near the end of the specification, similarly to other MP specifications that also deal with EE concerns (such as MP JWT: https://download.eclipse.org/microprofile/microprofile-jwt-auth-1.1.1/microprofile-jwt-auth-spec.html#_recommendations_for_optional_container_integration). Also, we should make the sentence more concrete. Something like:

Recommendations for Optional Container Integration

This section describes the expected behaviors when the implementation runs in a Java EE container.

Asynchronous

Threads that are servicing @Asynchronous invocations should, for the duration of the invocation, have the correct security context and naming context associated.

[Ladicek]

We should also add some TCK tests for this, in an optional way.

[Emily-Jiang]

related to #326

[Ladicek]

Submitted a PR. I didn't add a TCK test, because 1. it would have to be a separate TCK module, 2. even if I did that, I don't know how I would test this :-)