Take advantage of writev in batching handler

[ScottMansfield]

Go 1.8 has a new net.Buffers type which enables the use of writev on systems that support that syscall. The batching backend currently has a process where it writes all requests into a single buffer before sending it out the socket. This should be converted to use the net.Buffers type ([][]byte) in order to gain some efficiency by doing less copying.

There are probably more opportunities to use this type in more places, but they will come separately.