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.
Eventually we'll do the same with config callbacks, but now it won't affect this PR ;)