reproduction test for basho/riak_kv#475 (MR reduce with node down)

This test is based off one submitted by Alexander Gunin to the
riak-users mailing list as an issue reproducer.
[http://lists.basho.com/pipermail/riak-users_lists.basho.com/2013-January/010896.html]
use rt:systest_write instead of listcomp of riakc_pb_socket:put

Yay for using builtin utilities instead of hand-rolled hacks.
document rt:systest_write/5
Merge pull request #195 from branch 'bwf-mr-const-hash'