Design a unified oneDPL approach to asynchrony

[akukanov]

In this thread we tracked the proposal #1916, which goal was to design the approach and API for supporting asynchronous operations in oneDPL. The proposal is archived, see a1aaf97.

[akukanov]

The internal study group in the team have concluded that we should not proceed with the proposal. The RFC document (see #1916) describes the context and the reasons, as well as provides additional information from the study.