Some improvements for handling of many connections for MPM event:

- Process lingering close asynchronously instead of tying up worker threads
  (based on patch by Jeff Trawick).

- If the number of connections of a process is above

     threads_per_child  +  WORKER_OVERCOMMIT * (idle_workers - 1)

  (WORKER_OVERCOMMIT is fixed at 2, at the moment), or if all workers are busy,
  don't accept new connections in that process. Such a dynamic connection limit
  is necessary because we may have both async and non-async (ssl) connections.
  WORKER_OVERCOMMIT should be a config option.

- Don't count idle workers of not-accepting processes against MinSpareThreads,
  so that the parent will spawn new processes when necessary.

- If we receive a keep-alive request while all workers are busy, don't block
  but close the connection immediately so that the client will re-connect to a
  different process.

Related changes:

- Log what is going on at trace loglevels.
- Remove the bypass_push poll type flag, this code cannot be hit anymore
  (if it ever could?).
- Add some macro helpers for dealing with timeout queues.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1137358 13f79535-47bb-0310-9956-ffa450edef68