fix riak_kv_mrc_pipe:cleanup_* for dialyzer
fix riak_kv_mrc_pipe:cleanup_* for dialyzer

riak_kv_mrc_pipe.erl:825: The call riak_kv_mrc_pipe:cleanup_sink(Sink::'undefined' | {pid(),reference()}) breaks the contract (#mrc_ctx{}) -> 'ok'
riak_kv_mrc_pipe.erl:843: The pattern {'mrc_ctx', _, _, _, Sender, _, _} can never match the type 'undefined' | {pid(),reference()}
riak_kv_mrc_pipe.erl:854: Function cleanup_timer/1 will never be called

andytill/riak_kv