Add support for chunked replies.

The code is imported from dCache, but rewritten to follow the
structure of Netty's ChunkedWriteHandler and generalize to allow
other chunked replies that ReadResponse.

We cannot use Netty's ChunkedWriteHandler directly as we would
like to support out of order delivery of xrootd replies and
also do a limited form of read-ahead.

Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5146/
Add option for zero copy reads

I observe a 40% increase in read throughput when enabling zero-copy
operations.

There are however downsides to zero-copy IO:

- All disk I/O is pushed down into the thread pool used for
  network I/O.

- The read response is no longer broken into chunks, which means
  that other xrootd responses cannot be interleaved with a large
  read response.

The throughput improvements are however too large to ignore. With
zero copy enabled we are beating SLAC xrootd on my Mac.

Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5156/