Remove Luke usage

[beerriot]

riak_pipe powers MapReduce queries now

[beerriot]

In reviewing other PRs, I was reminded that we can also remove the "mget" call from riak_kv_vnode. Luke-based MapReduce was the only thing that used it.

[slfritchie]

Bryan, if you'd create a new issue to remove the mget call, I'm fine with that.

No new failing EUnit failures, no significant changes to Dialyzer warnings. +1 to merge all 5 related PRs, many thanks!

[seancribbs]

[kellymclaughlin]

Can this be removed as part of this pr also?

[beerriot]

Good catch, @kellymclaughlin! Unfortunately, it can't be remove the {riak_kv, mapred_system} capability just yet. If we remove it, and then there is a cluster membership change before all nodes are running the version of Riak with it removed, a renegotiation will flip those old nodes back to legacy, which will then cause them to fail at MapReduce queries.

However, new nodes also shouldn't lie about what they're capable of. I've just pushed a patch to remove legacy from what new nodes announce. Eventually we'll get all nodes announcing pipe by default, and then we can remove it.

[kellymclaughlin]

Great point, I need to do that for my legacy keylisting pr as well.

[kellymclaughlin]

+1 to merge after last commit