appbase io_context support for coroutines

[heifner]

appbase executor currently does not support c++ 20 coroutines. Currently SHiP is using app().get_io_service() which by-passes the priority queue executor. appbase executor/priority-queue should be enhanced to allow for its use with coroutines.

[bhazzard]

Holding off on this until something causes it to rise in priority.

For future discussion... Greg pointed out this as an upcoming standard Execution library: https://en.cppreference.com/w/cpp/execution