Implement riak-admin handoff enable/disable cmds

This implements the following commands:

riak-admin handoff <enable | disable> <inbound | outbound | both>
    [[--node | -n] <Node>] [--all]

It also adds cuttlefish config values for
handoff.disable_[inbound|outbound], and implements callbacks so that
disabling handoffs via the CLI will terminate any currently active
handoffs in the pertinent direction.

Finally, some unit tests have been enabled/added in the
riak_core_handoff_manager module to test the functionality that
terminates active handoffs.