Bug #671: Several minor aufs issues

Bug #671: Several minor aufs issues

- Enhance performance by zero-copy writes, enabled by making the mem
  nodes reference counted.
- Implement ASYNC_CLOSE define, default to off.
- Remove unused aioFDWasClosed call
- Kill warning about event notification filedescriptors still
  being open on shutdown.
  Also optimizes slightly by initializing the squidaio layer early,
  avoiding the need to check if initialized in each and every call.

Forward port of 2.5 patches.