Implementation of riak-admin handoff [summary|details] commands. In
addition to riak_core_handoff_status (the main module) the following
changes were made to other modules in support of this functionality.
* Refactored riak_core_vnode_manager:handle_call({repair...}) for readability.
* Refactor riak_core_vnode_manager:determine_handoff_target/4 into a series of function headers
* Create riak_core_vnode_manager:all_handoffs
* Some additional fixes for refactoring of handoff_type/ho_type into
  single cohesive type from 07e32b0