Major improvement in concurrent processing for AB:

Major improvement in concurrent processing for AB:
- Enable non-blocking connects.
- Prevent quasi-blocking mode apr_recv (which would prevent AB from
  multiplexing over the entire descriptor set).
- Catch other fatal apr_recv() errors.

[This patch is slightly different than the one posted to the dev list,
but regardless thanks to the many people who reviewed this.]

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94782 13f79535-47bb-0310-9956-ffa450edef68