work in progress for graphite thread
fixup so we always use TCP_CORK.

We were not loading the right header file so we never built with
TCP_CORK. According to `man 7 tcp` we needed to include netinet/tcp.h
to get the TCP_CORK which we were not. (Maybe my fault when I
reorganized things.)

Anyway, since TCP_CORK was introduced in linux kernel 2.2 it is pretty
old, and since we currently only target linux, it makes sense to just
assume we will always have TCP_CORK.
make the disk_writer share counters with its parent socket worker