Feature/riak cli handoff enable

This adds CLI commands to enable/disable handoffs:

riak-admin <enable | disable> <inbound | outbound | both> [--node <NODE>] [--all]

If handoffs are disabled, any currently active handoffs in the specified direction will be terminated.

If the riak-admin "set" command is used to disable handoffs in a given direction via the newly exposed handoff.disable_inbound or handoff.disable_outbound settings, corresponding handoffs will also be automatically terminated.

This PR also includes some additional unit testing in riak_core_handoff_manager.
I'm hesitant to introduce this test into riak_core. We shouldn't allow any spurious failures if we know they exist. It took a long time to get rid of the existing ones.

What you can do instead is use a wait-for pattern. We do this a lot in riak_tests. You'd do something like this: https://github.com/basho/riak_test/blob/master/src/rt.erl#L624

Then call it with function that returns a boolean like so:
```erlang
Fun = fun() ->  0 =:= length(status()) end,
?assertEqual(ok, wait_until(Fun, 30, 1000)).
```