win32/AsyncIO.c:onIOComplete(), awaitRequests():
  Avoid completion table overflow through the use of
  a semaphore. Worker threads (the producers) wait for a free
  event slot to appear, the RTS thread (consumer) signal slot
  availability once the completion event has been de-queued and
  processed.

  This bug only manifests itself in 'massively concurrent' code
  (>200 threads), where the threads simultaneously engage in IO
  through the async IO layer. conc023 and conc036 in the regression
  test suite are examples of such.

Merge to STABLE.