psgi_rproxy: standardize on 64K I/O size

psgi_rproxy: standardize on 64K I/O size

Much of our bulk I/O in DS and IO.pm already uses 64K buffers
(matching the Linux default pipe size) instead of 16K buffers.
While larger I/O sizes can result in more work for the malloc
implementation, they can also reduce syscall and Perl function
call overhead to reduce CPU usage and improve throughput on long
fat networks (LFNs).  Furthermore, standardizing on larger sizes
ought to reduce fragmentation since malloc can avoid splitting
existing buffers used for bulk I/O we do in other places.