cleanup claimant code around ring resizing

adresses review comments:
  * removes unneeded clauses in filter_changes_pred/4
  * add comments in several places to clarify behaviour
cleanup riak_core_ring code around ring resizing

address review comments:
  * add comments and typespecs, fix typos
  * refactor reschedule_resize_transfers/3
  * refactor resize_transfer_status/4
  * simplify future_index/4 a bit
fix guarded handoff_manager test that no longer compiles

NOTE: test still fails but that is unrelated to interface changes
that broke compilation
bring back determine_handoff_target in vnode mgr