Async upcoming (#6585)

* Merge upcoming async with current.
* Various improvements to selectors (mostly docs).

Two changes to highlight:

* Renamed ``setEvent`` to ``trigger``
* Reused setBlocking from nativesockets.
* Various changes/fixes to asyncdispatch after upcoming merge.
* Make some attempts to be compatible with older selectors.
* Reuse epoll module in ioselectors_epoll.

tests/testament/categories.nim

@@ -205,7 +205,6 @@ proc ioTests(r: var TResults, cat: Category, options: string) =
 proc asyncTests(r: var TResults, cat: Category, options: string) =
   template test(filename: untyped) =
     testSpec r, makeTest(filename, options, cat)
-    testSpec r, makeTest(filename, options & " -d:upcoming", cat)
   for t in os.walkFiles("tests/async/t*.nim"):
     test(t)