Asynchronous stage in software pipeline (#80)

* Add RFC: Asynchronous stage in software pipeline

* tweak

* typo

* Explain what async_scope is for

* another typo

* add authors

* typo fix, place `async_commit_stage` consistently in the examples

* stage -> queue

* remove a paragraph since we now talk about more general semantics

* change the definition of async_stages annotation from index to stage itself

* update doc for stage -> queue transition, talk about what is meant by queue

* make async_commit_queue scope annotation, rather than intrinsic

* fix in code example

* change async_wait to annotation too