msg/Pipe: move setting of socket options into a common method

Signed-off-by: Sage Weil <sage@inktank.com>
msg/Pipe: allow tuning of TCP receive buffer size

Performance tests on high-end machines have indicated the Linux autotuning
of the receive buffer sizes can cause throughput collapse.  See bug
#2100, and this email discussion:

   http://marc.info/?l=ceph-devel&m=133009796706284&w=2

Initially default to 0, which leaves us with the default.  We may adjust
the default in the future.

Signed-off-by: Sage Weil <sage@inktank.com>